When using more than one IRS2092 board, it's important to set the switching frequencies very accurately, or make sure that they're at least 25KHz apart to avoid audible inter-channel interference.

I replaced the 270 ohm resistors with a multi-turn trimpot and set all seven channels switching frequencies to be within <1Hz of each other using a cheap DMM with frequency meter (Victor VC921 with a 1.5 Kohm resistor in the positive probe). I did this on a test bench setup, with inputs shorted and an 8 ohm load on the outputs - I used 10W resistors, but be careful that the inputs really are shorted or they may start smoking!.

Important to set frequencies after everything has reached normal operating temperatures as they drift a bit; and of course I did all of this before installing the phase inverters or putting the modules into the Baking Tray enclosure - it would have been impossible afterwards.

One installed and 'run in', I re-checked the switching frequencies on the speaker outputs and all channels were between 399.6 and 400.2 Hz - I think a 0.6Hz difference is close enough!