I mounted a full-fledged 5-channel amp under the driver's dash. I went with the Alpine MRX-V70 and used a piece of 1/2" MDF to mount it to the steering column supports. I still have full tilt on the steering wheel and my feet never hit the amp or wiring when driving/shifting.
Check out my audio build thread
for some ideas, my second post shows some pics of the amp mounted.
Just getting something in there to power the whole system will make an enormous difference. The Kicker sub you have is being terribly under-powered by the factory amp so your bass will be a lot more punchy and less muddy. The Infinity speakers you have will also benefit from the added power and crossovers you can use to tune the sound.