You don't need to even know the input impedance of the poweramps unless you're filtering them passively or using a tubed preamp with high output impedance which may not be able to drive both amps well. You'll probably need gain controls on at least one pair of amp channels unless you're using identical amps...as in splitting a stereo amp 'vertically' on one speaker. Said another way, unless all amp sections have identical voltage gain (which is NOT the same as sensitivity for full power), you'll need gain controls on the amps with higher voltage gain because you'll need to turn them down in volume.
