I just replaced, today in fact, my HU with a Pioneer DEH-80PRS. Lemme tell you, it is a friggin miracle for the Mach 460. I too was about to throw out some major cash on a whole amp/speaker upgrade when I discovered this unit. It is beyond tweakable and believe it or not, makes those stock speakers really shine! It turns out the HU that I had in there was just not a good match for the Mach 460. It was a Kenwood DPX-300u.
NO JOKE, this is a top of the line, albeit 2 years on the market already, HU that is crazy expandable for whatever system you can imagine. I'm blown away. Got it on amazon for $80 less than everywhere else.