The master section on Harrison Raven includes 2 mix buses, P1 and P2.
P1 Mix BusEdit
We modded the summing amp in P1 by swapping out the old 5534 opamps and replacing them with Texas Instrument Burr-Brown OP16xx?? (insert proper chip # here). These chips came only in 8-SOIC packages, so we had to mount them on small 8-DIP adapter boards from SparkFun electronics to install into the DIP sockets on the summing amp board.
P2 Mix BusEdit
Currently the P2 mix bus is only half-working. The right channel is silent. We've swapped the leads on the P2 volume pot and determined that it wasn't the problem - it only swapped which channel was silent. So we know the problem in this signal chain is pre-volume pot.
I also recall swapping the opamps for the summing amp on P2, but this had no effect. So the problem is not the opamps in the summing amps.