Yo!
I know, I know, you've said you don't want a sub in the boot...
but have you considered a
small (and affordable) Active Sub for the boot? Give it a thought because they truly are fantastic.
Small being only an 8" / 20cm cone and Active meaning its got a built in and sensibly powered amp. Being a smaller cone too, they're ultra responsive and you don't get that
delay you often hear with bigger cones.
I've got a Sony Xplod XS-AW200 and its fantastic. Beats out some great levels if you want it to but I have it tuned to give a 'responsive warm punch', but what's really good about it is that I can control the level of the sub independently from all other speakers via the head unit (you'll have to check your new one to be sure, but the majority have this feature these days).
The benefit of a boot sub no matter how big or small, is that you can let it deal with ALL of the bass, and the rest of your speakers focus solely on the mids and highs; and because your relieving the car speakers of the bass, they're not struggling and straining so the quality of everything else flies up - along with the volume if you desire. So if you could get your hands on a second hand Xplod XS-AW200 for £30/£40 - its a cheap and hugely effective upgrade.
Also - for the long trips where I need ALL my boot space, its just a case of removing the phono's and one plug and out it comes as I just secure it down with bungie cables. I have the the speaker facing the wheel arch too so it just looks like an inconspicuous grey box, and not a show off sub.
My Sony Xplod (not my very one, but an example of how cheap they go for):
http://www.ebay.co.uk/itm/Sony-Xplo...7?pt=UK_In_Car_Technology&hash=item2572cf6b07
Some Alternatives (these two are good because they're angled for your back seat, i.e. room saving):
http://www.ebay.co.uk/itm/Blaupunkt...5?pt=UK_In_Car_Technology&hash=item337860fcd3
http://www.ebay.co.uk/itm/VIBE-LITE...6?pt=UK_In_Car_Technology&hash=item27bbdc975e
Under-seat Subs do an ok job I believe but as said, its mostly just like having a speaker under your seat - I'd be surprised if the overall bass in the car was greatly improved. Your bang for buck comes from the stuff above as under-seat subs cost about the same if not a lot more; example below:
http://www.ebay.co.uk/itm/Blaupunkt...3?pt=UK_In_Car_Technology&hash=item4ab9bf1de5
Another thing to consider with under-seat subs too is, your front seats - do you have a 3 door car and are the front seats moved forward and backwards all the time? Not sure how an under-seat sub would deal with that.
Hope that helps anyway matey,
Rhauri