Are they typically worth it? I am absolutely not interested in banging choons, preferring hi-fi nice reproduction, but I do wonder if it'd help bring the bass out over the road and engine noise.

Or should I just add sound insulation instead?

my Rel Strata III would just about fit on your passenger seat?

I put a small Blaupunkt sub in my last car it and was loads better.

Depends on whether you have an aftermarket stereo or not??

With the Kenwood I had, you could fiddle around with all the low bypass filters, so the 4 speakers in the car didn't receive any real bass and this was all sent to the sub to handle. Made the whole thing sound much better.

With a factory stereo I doubt you get the option to set-up any filtering.

I've got a Pioneer with a subwoofer out and fair few settings allowing fiddling. Was yours expensive?

Mine was the older version of this (and at the time cost £150) but luckily had a metal grille over the cone rather than those stupid metal rods.

http://www.bluespot.co.uk/car-audio.aspx/emb800a-speaker

Pretty punchy for a 1 ft square box.

This one looks a bit less 'boy-racer' I reckon.

http://www.bluespot.co.uk/car-audio.aspx/thb210a-speaker

Not sure where I'd fit that apart from the boot. Wouldn't the boot muffle the sound a lot? It's a saloon.

If you do it right, then they're worth it. Don't just pop to Halfords for a 15" sub & amp combo – they're generally built in bandpass/ported enclosures to give massive bass at certain frequencies, thus sounding "good" in the store but generally a bit patchy in your car.
If you're just after tight bass over a wide range of frequencies, get yourself an 8" or 10" sub and build an enclosure out of MDF/fibreglass with a suitable internal volume for the sub you have. There are plenty of calculators on the internet that'll work it out for you, like this one here.

I did see a super-slim one that fit under the seat.. could be an other option..?

http://www.dynamicsounds.co.uk/kenwood-kscsw01-powered-subwoofer-under-seat-subwoofer-150w-p-15625.html

what are the speakers you have?

If i was doing another car audio project (i'm not – the last ones were like drug habits!!) I'd be spending my money on a nice head unit. and the majority of the budget on nice set of components in the front and some 8 inch rounds in the back. possibly no amps.

Mine went in the boot of my hatchback – a friend of mine had one in a Passat (saloon) and it was pretty loud.

Most car subs are designed to go in the boot, although Kenwood used to do one that was very shallow and would fit under the front seat of a lot of cars. Can't imagine it being very punchy though.

EDIT – you found the under-seat one, then!! Bit slow….

The speakers are the OEM VW Passat jobs. I think that they are components which puts me off a bit, since the tweeters look like they are fairly custom to fit in a funny shaped panel in the top of the door. Could be worth replacing the bass units tho instaed, I dunno. Perhaps insulatino is the way forward, but it could be a right old faff.

You should check the dimensions carefully before getting a sub to slide under your seat.

I bought a CD changer that should have fitted under my seat, but didn't by about 5mm because of some peculiarities under the seat I hadn't considered…..

I thought Passats were meant to be refined and quiet? Maybe that little diesel is struggling a bit so make a bit of a racket?

Subs under my seats (factory fitted OEM) that do kick out some nice bass – I'm sure they'd fit under a Passats too. Don't forget a crossover though.

I looked into this on some of the car audio forums. They reckoned that the Elemental Designs 11Kv was the best option for a smallish box & driver and not wanting huge amounts of bass.

As for fitting them under seats, it's a bit of an ask. The phsyics demands a decent sized box. The best place I could think of to locate one in a hidden way was behind the glove box – there's a fair bit of room under the glove box / behind the knee bar in the cars I've looked at (Golfs).

It's not bad, but there's still road noise. The engine's pretty much inaudible on the motorway but it could be masking some frequencies of the music. In my world, 140bhp is not little 🙂

Head unit has the crossover.

I'd pop into a local fitters – I usually find most are brilliant for advice. Definitely go under the seats if you can – boot installs are a PITA and nick half the space.

I've had acoustic shelves built into hatchbacks before but this doesn't work on a saloon or estate.

Otherwise a speaker upgrade and an amp might do the trick.

Do you really NEED 140bhp when a perfectly "safe" 110bhp version is available 😉

There's a limited pocket behind the wheel well in the boot that I could happily lose. Perhaps big enough for a custom enclosure. But the seats seem to be designed to insulate a lot of road noise, so could affect things.

Investigating insulation I think. The Prius would benefit from it too as road noise is about the only noise there is on it so it could end up super quiet.

That'll start costing big money. Definitely check the under seat option first – not much room under mine but fair sized subs are stuck under them.

Roll of generic (ie non-car audio branded but the same) insulation costs about the same as a cheaper sub.

But will also check the other options mentioned here.

boot installs are a PITA and nick half the space.

eh?

The sub in my car was < 1 foot cubed……and installing it was 1 power & 1 signal cable. Took about an hour to route through the interior + perhaps 15 mins to set up all the low/high bypass filters.
I think there was an earth that went to one of the rear lights too, thinking about it……

My a4 has an oem sub in the boot behind the wheelarch which adds a bit of depth. Worth investigating if an audio upgrade was an option on your car and something you could get on eBay?

Interesting, I could look for the OEM option. One from an Audi might work.

I loved having a sub, much more depth to the music if the standard system is badly designed/cheap, but it needs to be part of a whole system – no point putting a nice sub in with 4w front door speakers 🙂 Mine was two 8" 300Wers, one on each end of a 1" wall thickness PVC water main pipe about 1m long. Not ideal, wasnt exactly "designed" but certainly worked. Actually worked better with the drivers in opposition rather than in unison (which is apparently the normal method).

let me know if you need pics of the audi one in situ