UK car audio comparison

Best Slim Car Subwoofers
in 2026.

A practical comparison of car subwoofers using enclosure type, power, dimensions and the installation space they need.

Prices and stock should be checked on the retailer page before purchase.

Our approach. We compare the specifications shown on the retailer product pages and explain who each model suits. This is not a hands-on test.

Our pick

In Phase USW300 300W Underseat Ultra Slim Compact Active Subwoofer System Die-Cast Aluminium

A active subwoofer choice for adding more weight to the system.

Check price

Full specifications

Our picks compared

This table includes every verified specification available for the products in this roundup. Check the retailer page for the latest price, stock and fitment details.

Product	Price	Availability	Brand	Model	Warranty	Product Type	Subwoofer Design	Size	Listed Power	Rms Power	Type	Feature	Shipping Weight	Full specifications
In Phase USW300 300W Underseat Ultra Slim Compact Active Subwoofer System Die-Cast Aluminium	£99.99	In stock	In Phase	USW300	2 Year Warranty	Car Subwoofers	Active	Not listed	300W	Not listed	OEM Headunit Connection, Underseat Subwoofer	Built In Amplifier	5 kg	Product Detail: 4 Ohm Size Detail: Under Seat
Kicker 48CWRT672 CompRT 6.75" Thin Profile Dual Voice Coil Subwoofer - 2 Ohm	£119.95	In stock	Kicker	48CWRT672	Not listed	Car Subwoofers	Not listed	Not listed	300W	Not listed	Not listed	Not listed	Not listed	Impedance: Optimized for a 2-ohm impedance, the COMPRT subwoofer is compatible with a wide range of amplifiers, Power Handling: Despite its compact size, the 48CWRT672 can handle impressive amounts of power, allowing it to deliv Size Detail: 8 inch
Phoenix Gold ZX18PDF 8" 500W MAX Slim Passive Subwoofer Box	£159.99	In stock	Phoenix Gold	ZX18PDF	Not listed	Car Subwoofers	Passive	8 inch	500W	250W	Not listed	Not listed	Not listed	Impedance: Optimised for efficient power delivery and compatibility with various amplifiers Size Detail: 8 inch
Phoenix Gold ZXT10D4 700W Max Power 10″ Slim Subwoofer	£169.99	In stock	Phoenix Gold	ZXT10D4	Not listed	Car Subwoofers	Passive	Not listed	700W	Not listed	Not listed	Not listed	Not listed	Impedance: 2×4 Ω Frequency Response: 30Hz – 500Hz Power handling (RMS): 350W Power handling (MAX): 700W Sensiti Mounting Depth: 95 mm Mounting Diameter: 232 mm Outer Diameter: 264 mm Speaker Grills: No Unit: Piece Weight: 4 Speaker Design: Component
Phoenix Gold ZXT12D2 12" 800W Max Power Slim Subwoofer	£179.99	In stock	Phoenix Gold	ZXT12D2	Not listed	Car Subwoofers	Passive	12 inch	800W	400W	Not listed	Not listed	Not listed	Impedance: 2 x 4Ohm Mounting Depth: 95mm ZXT12D2 Features: Stiff, lightweight fibreglass cone Buytl Rubber Surr Sensitivity: 82 Speaker Design: Component
BLAM RELAX BL-MSP25P MSP 25P Extra-Slim Passive Underseat Car Subwoofer	£260.00	In stock	BLAM	BL-MSP25P	Not listed	Underseat Car Subwoofers	Passive	Not listed	Not listed	Not listed	Not listed	Not listed	Not listed	All listed fields are shown

Product notes

What each pick is best at

Best overall bass upgrade

In Phase USW300 300W Underseat Ultra Slim Compact Active Subwoofer System Die-Cast Aluminium

A active subwoofer choice for adding more weight to the system.

What we like

Lowest listed price in this comparison at £99.99, £19.96 below Kicker 48CWRT672.
The only pick here listing Active design, giving it a clear difference from the other 5 options.
Its active design includes amplification in the product, unlike passive options that need a separate amp.

What we don't like

Unlike some alternatives, the listing does not show a comparable design figure, so matching it to the rest of the system takes more checking.
The enclosure and underseat or boot dimensions still need checking, even where the product is described as compact.

Price	£99.99
Availability	In stock
Listed Power	300W
Model	USW300
Subwoofer Design	Active
Warranty	2 Year Warranty
Brand	In Phase
Feature	Built In Amplifier
Product Detail	4 Ohm
Product Type	Car Subwoofers
Shipping Weight	5 kg
Size Detail	Under Seat
Type	OEM Headunit Connection, Underseat Subwoofer

Check CarAudioCentre.co.uk

Best for compact installations

Kicker 48CWRT672 CompRT 6.75" Thin Profile Dual Voice Coil Subwoofer - 2 Ohm

A bass subwoofer choice for adding more weight to the system.

What we like

At £119.95, it costs £19.96 more than In Phase USW300, so the extra spend is tied to the specification and fitment choice.
Its listed 300W power matches 1 other pick, making like-for-like installation planning easier.
The product type gives a clear starting point for deciding between a compact bass upgrade and a separate amplifier setup.

What we don't like

It is £19.96 above the cheapest pick without a clearly unique listed specification to explain the difference.
The enclosure and underseat or boot dimensions still need checking, even where the product is described as compact.

Price	£119.95
Availability	In stock
Listed Power	300W
Model	48CWRT672
Brand	Kicker
Impedance	Optimized for a 2-ohm impedance, the COMPRT subwoofer is compatible with a wide range of amplifiers,
Power Handling	Despite its compact size, the 48CWRT672 can handle impressive amounts of power, allowing it to deliv
Product Type	Car Subwoofers
Size Detail	8 inch

Check CarAudioCentre.co.uk

Best for boot-based systems

Phoenix Gold ZX18PDF 8" 500W MAX Slim Passive Subwoofer Box

A 8-inch passive subwoofer choice for adding more weight to the system.

What we like

At £159.99, it costs £60.00 more than In Phase USW300, so the extra spend is tied to the specification and fitment choice.
The only pick here listing 8-inch size, giving it a clear difference from the other 5 options.
The passive design leaves amplifier choice open for buyers already planning a separate bass channel.

What we don't like

It is £60.00 above the cheapest pick, so the higher price depends on whether the distinct 8-inch size suits the installation.
A separate amplifier and suitable wiring are required, unlike the active alternatives.

Price	£159.99
Availability	In stock
Listed Power	500W
Model	ZX18PDF
Rms Power	250W
Size	8 inch
Subwoofer Design	Passive
Brand	Phoenix Gold
Impedance	Optimised for efficient power delivery and compatibility with various amplifiers
Product Type	Car Subwoofers
Size Detail	8 inch

Check CarAudioCentre.co.uk

Best for factory stereos

Phoenix Gold ZXT10D4 700W Max Power 10″ Slim Subwoofer

A passive subwoofer choice for adding more weight to the system.

What we like

At £169.99, it costs £70.00 more than In Phase USW300, so the extra spend is tied to the specification and fitment choice.
The only pick here listing 700W power, giving it a clear difference from the other 5 options.
The passive design leaves amplifier choice open for buyers already planning a separate bass channel.

What we don't like

It is £70.00 above the cheapest pick, so the higher price depends on whether the distinct 700W power suits the installation.
A separate amplifier and suitable wiring are required, unlike the active alternatives.

Price	£169.99
Availability	In stock
Listed Power	700W
Model	ZXT10D4
Speaker Design	Component
Subwoofer Design	Passive
Brand	Phoenix Gold
Impedance	2×4 Ω Frequency Response: 30Hz – 500Hz Power handling (RMS): 350W Power handling (MAX): 700W Sensiti
Mounting Depth	95 mm Mounting Diameter: 232 mm Outer Diameter: 264 mm Speaker Grills: No Unit: Piece Weight: 4
Product Type	Car Subwoofers

Check CarAudioCentre.co.uk

Best for stronger low end

Phoenix Gold ZXT12D2 12" 800W Max Power Slim Subwoofer

A 12-inch passive subwoofer choice for adding more weight to the system.

What we like

At £179.99, it costs £80.00 more than In Phase USW300, so the extra spend is tied to the specification and fitment choice.
The only pick here listing 12-inch size, giving it a clear difference from the other 5 options.
The passive design leaves amplifier choice open for buyers already planning a separate bass channel.

What we don't like

It is £80.00 above the cheapest pick, so the higher price depends on whether the distinct 12-inch size suits the installation.
A separate amplifier and suitable wiring are required, unlike the active alternatives.

Price	£179.99
Availability	In stock
Listed Power	800W
Model	ZXT12D2
Rms Power	400W
Size	12 inch
Speaker Design	Component
Subwoofer Design	Passive
Brand	Phoenix Gold
Impedance	2 x 4Ohm Mounting Depth: 95mm ZXT12D2 Features: Stiff, lightweight fibreglass cone Buytl Rubber Surr
Product Type	Car Subwoofers
Sensitivity	82

Check CarAudioCentre.co.uk

Best for flexible installation

BLAM RELAX BL-MSP25P MSP 25P Extra-Slim Passive Underseat Car Subwoofer

A passive subwoofer choice for adding more weight to the system.

What we like

At £260.00, it costs £160.01 more than In Phase USW300, so the extra spend is tied to the specification and fitment choice.
Its listed Passive design matches 3 other picks, making like-for-like installation planning easier.
The passive design leaves amplifier choice open for buyers already planning a separate bass channel.

What we don't like

It is £160.01 above the cheapest pick without a clearly unique listed specification to explain the difference.
A separate amplifier and suitable wiring are required, unlike the active alternatives.

Price	£260.00
Availability	In stock
Model	BL-MSP25P
Subwoofer Design	Passive
Brand	BLAM
Product Type	Underseat Car Subwoofers

Check CarAudioCentre.co.uk

Buying guide

What to check before ordering

Start with the space available, then compare RMS power, enclosure type and input options. A factory stereo may need high-level inputs or a line-out converter, while a passive subwoofer needs a separate amplifier.

Use the model number as the final reference. Retailer images and prices can change, and vehicle fitment should be confirmed for the exact make, model and year.

Best Slim Car Subwoofersin 2026.

In Phase USW300 300W Underseat Ultra Slim Compact Active Subwoofer System Die-Cast Aluminium

Our picks compared

What each pick is best at

In Phase USW300 300W Underseat Ultra Slim Compact Active Subwoofer System Die-Cast Aluminium

What we like

What we don't like

Kicker 48CWRT672 CompRT 6.75" Thin Profile Dual Voice Coil Subwoofer - 2 Ohm

What we like

What we don't like

Phoenix Gold ZX18PDF 8" 500W MAX Slim Passive Subwoofer Box

What we like

What we don't like

Phoenix Gold ZXT10D4 700W Max Power 10″ Slim Subwoofer

What we like

What we don't like

Phoenix Gold ZXT12D2 12" 800W Max Power Slim Subwoofer

What we like

What we don't like

BLAM RELAX BL-MSP25P MSP 25P Extra-Slim Passive Underseat Car Subwoofer

What we like

What we don't like

What to check before ordering

Best Slim Car Subwoofers
in 2026.