Receiver working as pre-amp, TURNS OFF, whenever Second Zone/Speaker B goes loud

In my living room, I have a 9.1 Pioneer Elite Receiver (at the time, their top of the line, largest PSU version), as my Home Theatre, and my pre-amp for my 2-ch audio with a Classe’ amp.

It power 5.1 in one room, and 2-ch in another room. What’s happening is that whenever I am listening to music, or watching video, IF I have the second room/zone ON, and if I get the volume high to -20dB or higher, on LOUDER passage, the Amp shuts down! Doing the same with 2-ch (through the amp), or 5.1 (2 from Classe’ the other 3 from receiver), it has NO ISSUES! Could go 0dB or +5dB, over 100dB volume level.

Question is, what’s the main cause? First thought is that Amp does NOT have enough power to feed all channels, despite having supposedly 4 more channels to drive (as front channels have their own amp), or that the LONG WIRING to the other room speakers are causing some higher load, or other issue? Could it be speaker terminations? or what else could be causing this?