Depends on the year, but as far as I know they all have at least 3... the 04 had the tweeter amp (4 ch) connected as part of the head unit and each of the back amps supplied power for it's respective side (2 ch each).
I think the 98 had the tweeter amp on the floor behind the head unit near the airbag controller. There is a converter (it has sliders and such) that you can hook up inline to the rear amps to alter the signal going to the amps. Those amps require a little more than line levels.
The whine you are starting to get may be from a faulty ground somewhere for that particular amp. Can you tell if it's just on one side or the other? If it's on both, it's probably your head unit or the wiring.
From what I know, you are lucky to have a 98 Mach system, the 04 didn't have a harness for about 2 years and it was completely different from the other years. Cost me 300 in labor to completely rewire the car front to back in 04.