A bit late I know, but just seen this and thought I'd add my bit.
I ran an external power amp for the main front channels off the pre-outputs for those channels, so I had the main amps being unused and they had straightforward input sockets (RCA) available, as the bridging jumpers had been removed to connect the external power amp. But what to use these amps for?
I wanted to connect the rear surround output to these so that I could run 4 rear surround speakers, albeit with both Ls and both Rs playing the same signal, but amplified separately. Trouble is that once you plug an RCA jack into the rear channel output sockets, the internal amp is automatically disabled. BUT, it seems to be a physical switch and if you only gently push them in - not all the way, you can make good contact, without disabling the internal amp.
So, I ran a short interconnect (no earth connection, just grounded at one end) into each main front amp input jack, from the corresponding rear surround output jack - only gently pushed in till it made contact, but leaving the internal amp also processing the same signal. Worked fine for some time until I replaced the amp.
Or, previously I similarly wired the stereo subwoofer outputs into the spare main front inputs and ran passive subs. The mono sub output was also utilised into a powered sub. This was easier as there was no need for such careful insertion of the jacks as in this case there's no chance of disabling any internal amp.
Just some ideas.