You can replace the deck in a mach460 system. It requires an adapter. I usually run RCA level into the Mach amps and add a line driver/preamp to it. If the deck is 4V or more, you will only have a slight drop in volume from the factory setup. You can run speaker level into the amps as well but IMO, it does not sound as good. It may play louder, however.
Are you running a sub now? If so, I would pick a deck that has built in crossovers so you can run highpass only through the mach system.