crustycoot
his are monoblocks. even easier, smaller single channel device. I would definitely look for room inside, drill a hole, add a high quality volume control to each amp.
I've added on/off switches to a few things, but if thinking about resale ....
I somehow doubt OP will mess with his amps, so an external box with a high quality single channel volume control would do it. ALPS seem to be liked by many makers.