I purchased this amp as a repair/resell item. Amp symptom was loss of audio in front channels.

I powered up the amp and while monitoring the output I exercised the control setting. The front channel 3 position crossover switch had a dirty connection causing loss of audio. 

I used 401B - Nu-trol Control Cleaner to clean and lubricate the switches as well as the gain POTS. Amp was retested and passed clean signal to the output with no loss of signal.