You seem to have it hooked up right and understand how it should work. I think it needs service. I wonder if the 2/4 channel switch on the back has failed, is very dirty, or bypassed by a previous owner to get a louder 2 channel system when quad failed commercially. If you can get help from the person you bought it from or another tech, that seems the logical step.
So You think the problem is in 4ch/2ch switcher on back side of amp?
It is not possible to get help from person I bought it. I bought amp from Canada and I live in Czech republic, as You can see on my pure english . It was long and expensive transfer.
Thans for helping - @par4ken , @Circular Vibes , @LuvMyQuad - Last word, is the amp broken and need service? Probably the 2/4 switcher?