Have you tried a sub in the boot (or trunk)? I want to fit one myself but I was concerned that the sound wouldn't reach the cabin.If you live in the Southern California area (Los Angeles to San Diego)... Al & Ed's can do it.
I had them take out the Bose system and replace it with Focal speakers, an Alpine PDR-V75 amp that has a 350 RMS watt output for the sub. They will be installing a thin-line 10" Focal stealth sub with a box where the spare tire currently goes, when I'm ready for it.
EDIT Ignore me, got mixed up which forum I was on, I have a boot sub in my Mazda 3 and its great. Though this was an MX-5 forum :S