Is it possible to extend speakers selector to 4 pairs?
You can interchange input and output selectors with some minor mods....
This way SUB becomes useless, i.e. 2 inputs and 4 outputs.
Or, just buy my mastering console monitor section
It has 127.5 db range in 0.5 db steps and more functions....
One more question... How did you implemented talkback function?
Is the CR level dimmed when TB is pressed?
There's talkback combiner, which mixing small electret mic signal
into playback L-R feed from external source (or same source as CR).
When TB pressed, CR level goes down by user-defineable value (0...-20dB)
and playback level dim'ed as well by 8 dB.
Talkback combiner has balanced 20k inputs and ground-compensated outs.
If TB pressed more than 4...5 sec, it latches; press again to off.
I will post schems and other info about TB combiner in few days.