Matt,

I'm no expert, (I'll leave that to the pros at La Jolla Audio) but my take is this.
You can replace a speaker, but if you take out the HU, you need to bypass the amps. My neighbor built some harness connectors to utilize the existing wiring for his 400E but the amps are pretty much of an integrated unit with the HU.
You can probably sell the the old system to someone here if you take it out carefully and don't destroy things. That should help pay for a nice 4 channel amp.

Good luck

__________________
Bryan Harter
1992 500E Smoke Silver
2002 CLK55 AMG Cab Silver/Black
2001 Harley Fatboy - Pearl White
2004 F150 FX4 Silver/Grey
1969 GTO Liberty Blue
1956 F100 Black