I replaced it and subsequently discovered this problem. After tracing things through using a signal generator and oscilloscope, I found U101 (4560) to be toast - with DC on all its inputs and outputs. The fault reported was that Channel A had no output with the amp configured in this way. The customer said that it gets used on bass duty, with the internal crossover and low cut filter activated on both channels, with the high output from the crossover feeding the hi-mid amp.