I have successfully moved the subwoofer channel to 88A input 1, thereby eliminating the buzzing sound on the left channel. In case anyone else wants to try, here are the steps. It will take approximately 15 minutes.
1. Swap the input connections, moving the LFE input to 88A channel 1, and the FL input to 88A channel 3. Easier with unbalanced connections than re-wiring the Phoenix block!
2. In the 88A bass management software, open the Routing tab, and re-name FL In 1 to LFE In 1, and LFE In 1 to FL In 1 (to avoid future confusion).
3. Route the signal from FL In 1 to Dirac 1, and turn off the routing for LFE In 1 to Dirac 3.
4. Open the LFE Mgt tab, and re-name FL In to LFE In, and LFE In to FL In (to avoid future confusion).
5. For the LFE In channel, set both the HPF and LPF to "Bypassed".
6. For the FL In channel, change both the HPF and LPF to "Bypass", and select the appropriate crossover settings for the left channel (100Hz and L-R/24dB/Oct in the example).
7. Using your AVR speaker test tones, test to make sure all channels are outputting sound properly.
8. Verify that no buzzing sound can be heard on the left channel.
Note that re-running the Dirac calibration is not necessary, because all of the configuration changes happen before Dirac, and the same channels are ultimately input to the same Dirac inputs.
Edit: And a special thanks to
, who suggested this channel swap back in post 13980. You always have great ideas, Sanjay!