“Tops are separate RL. Subs are on Aux 10 only with Y cable from Amp Ch 1-2 into Mix 10. Will try Ch 1 into mix 10 and Ch 2 into mix 9.”
Out of Mix 10 to the amp I assume. May seem pedantic, but if you’re thinking clearly about the signal flow, it helps enormously when fault finding.
As mentioned above, Mix 9&10 are permanently linked as stereo pair, so either run the subs in stereo or choose a mono output for those to avoid complications in your routing.
Try just one channel (mono) input first, sent to L&R, to check your balance, doing the output cable swapping one end at a time and checking again, to work out whereabouts in the chain the problem lies.
Note that with the subs in mono, all driven from Mix 10 any volume difference between them can’t possibly be due to the desk.