With all those extra boards (particularly the top red 'control' board) in there I would be surprised if it doesn't 'know' it is a 2 channel amp. In the datasheet it isn't explicit whether the Standby/Protect Control pin just turns off the amplifier output, or the 500ASPs power supply as well. From the block diagrams it would appear to be just the output stage, which would suggest the control board is probably holding this low permanently to disable the 500ASPs output. This could be overridden by just pulling out that pin from the connector (pin 3, connector P2), which would keep it high all the time. I can't tell from the picture if P4 (Speaker outputs) is connected at all on the 500ASP. My suggestion would be to pull out P2 from the 500ASP, then power up the amp and see if it plays normally. If so the control board obviously doesn't care that it is disconnected from the 500ASP. Next attach some binding posts to P4 and a RCA connection to pin 9 & 7 on P2, turn the amp on and see if it plays. If it does, reattach P2, pull out just pin 3 from that connector, solder on the RCA to the input board and permanently attach the binding posts! Although it looks like you may need to do some metalwork to open that up... It's strange, the input board appears to have facility to turn the RCAs into a balanced input to feed into the amp, but they have chosen not to populate it on any of the channels. Otherwise you could always ask the manufacturer if they would upgrade it to the 3 channel for a price?