It looks to me like the MPSS-4 should work just like the Sony switch if you just ignore parts of the wiring guide - ie don't use any wires from the switch to power/accessory on your empeg or other HU.

Having looked at the picture at Crutchfield, there does look to be a unified amp remote turn on output, even though the blue wire is depicted as an input on the installation sheet, it is physically located together with the RCA outs on the "output" side.

/Michael