I had mine upgraded but paid an ICE shop to do it.. didn't go for a sub as I didn't want to give up boot space.
Jup had the same thing, wanted sound with a bit of punch but did not want to sacrifice any space.
Now considering whether to get a small sub as even with decent mid-range speakers the bass isn't that good. Anyone got any suggestions?
I've upgraded the front with a FOCAL 5.25"/13cm PolyKevlar component set (did attunate the tweeter an extra -3dB to -9dB total) and for the low end I've used the original speakermounts of the parcel shelf. I like the sound to only come from the front, so putting subs in the shelf was fine for me.
First I've made the parcel shelf heavier with some lead-bitumen (those sound deadning stick-on panels -but then with lead in them) and made sure every surface the shelf touched was made rattle free with some draft-stopper tape. I've run with the parcel shelf original, but that rattles with every beat.
For subwoofers I found the Earthquake SWS6.5; a set of two 6.5" sub-oriented woofers with a different magnet structure, so they are very shallow -especially handy above the battery. They're made for free-air work, I found most other 6.5" 'sub' oriented woofers were not really made for free-air work.
Amp (Audison SRx4) was positioned under the parcel shelf, slant against the bit of metal holding the fuel-vapour-recovery stuff, facing forward (nice to adjust it
). Front two channels drive the front FOCALs full range, rear two channels are bridged (low pass drops to -24dB/oct) and drive the two subs in the shelf at a low-cut of ~<120Hz.
I do have to mention I have a open Ragazzon for centre silencer, so my idle-exhaust-hum does ruin a lot of low end actually. Adjusting the low-cut frequency higher can give a loud OUMPF but if set too high, every singer will sound like Barry White.
Sound is very clean and open with just that bit of low-support I wanted. It won't thump that they'll hear you 4 blocks away, but it'll give a very decent full-bodied sound. A dedicated sub-box would give a tighter bass, but it's virtually impossible to make a decent one without sacrificing (useful) space.
If really crancked open, something will rattle from behind that could be the shelf, or well be some interiour bits, or numberplate, or stuff in the boot, kidneys, teeth ect -at that sound level it's just too loud to properly hear what is rattling. But I made it for clean sound, not SPL-contests -and I still have all of my boot space...all of it!