get a pair of headphones, and then goto radioshack and buy an adapter to convert the 1/8 plug on the end of them to stereo rca connectors. either that, or hack the end off of the phones and solder rcas onto the cable.
plug these into your hu's outputs, and find out the exact spot where your hu starts clipping. youll know youre there when it starts distorting.
you can use these the same way with any preamp-level processor.
this will allow you to get the maximum amount of CLEAN voltage to drive your amps input stages!
have fun, start soldering!