Line level from stock HU? Or install new HU? Options?

I have had my TSX for about 6 months now, and love the car, though I can't stand the stereo. I have already taken out the 6x9's and built a sub box which form-fits inside the spare tire well. The subs are powered by an amp which is fed the amplified signal from the stock head unit/amp(s). While the bass is a lot cleaner, I want to revamp the rest of the system, but I really do NOT want to feed additional amps the already amplified signal coming from the crappy stock system, or use a line-level converter. Does anybody know about the signal going from the stock HU to the stock amplifier(s), as in, which wires are which, and whether the low-level output from the stock HU is suitable for connection of aftermarket amps? I would like to keep the stock 6-disc HU, if possible, but not use any stock amplification. It would be a pain to try to take that HU out of there and replace it, as the climate controls, etc. are all integrated. Is the stock HU interfaced to any other functionality in the car, such that, if I remove it, will it cause any problems? I don't have the navigation system, BTW.
Any info would be appreciated.

If worst comes to worst, I will either rip out the stock HU and fabricate some housing in its place & put my Alpine in there, or just leave the stock HU in, but disconnect it, and install my Alpine into the compartment below somehow, if that is possible. Are there any aftermarket HU install kits that would work with the TSX?