Thanks Al, unfortunately the subwoofer already only has a groundless connection (factory default) so I know it's not part of the problem.
I cannot use balanced connections to the amps because I need the low frequency separation at the subwoofer (so there is only the preamp-RCA-subwoofer-RCA-poweramp route or putting the subwoofer in the chain after the poweramps using the speaker wires (IMHO worse than RCA); this 80 Hz split yields a big performance improvement for the speakers similar to the effect of bi-amping. Not happy that the sub doesn't support balanced in-out for this purpose, but things are what they are ...
At different times of day my issues with the hum are almost unnoticeable so one of the next steps will probably be to get a separate line (or better two) from the main distribution box to the music system (I don't think that this can be avoided).
Spending money on power related improvements so far has been worth it (PowerCell performing better day to day after settling down).
I cannot use balanced connections to the amps because I need the low frequency separation at the subwoofer (so there is only the preamp-RCA-subwoofer-RCA-poweramp route or putting the subwoofer in the chain after the poweramps using the speaker wires (IMHO worse than RCA); this 80 Hz split yields a big performance improvement for the speakers similar to the effect of bi-amping. Not happy that the sub doesn't support balanced in-out for this purpose, but things are what they are ...
At different times of day my issues with the hum are almost unnoticeable so one of the next steps will probably be to get a separate line (or better two) from the main distribution box to the music system (I don't think that this can be avoided).
Spending money on power related improvements so far has been worth it (PowerCell performing better day to day after settling down).