Yamaha DSP-A1 Amplification in L-C-R Channels

I bought a Yamaha DSP-A1 when it first came out and it provided many years of home theater enjoyment. About 4 years ago, I bought some large L-C-R speakers, so I started to use the DSP-A1 as a preamp, routing the output of those 3 channels to more powerful external amps.

I recently bought a proper preamp so I disconnected the DSP-A1 and was hoping to use it as a stand-alone amp/preamp again. The DSP-A1 seems to have "forgotten" how to provide power to the L-C-R channels. I think the years of running it without any load on the L-C-R outputs might have blown something in those circuits.

It this an easy fix? The surround and "front" channels are still good. It seems a waste to discard such a classic unit. Any ideas on a fix will be most welcomed!