Preamp/Tone board is fine, it is not supposed to have meaningful gain, it is there for tone control, and some filtering for the sub.
I would be eyeballing the power amp and possibly IC6 on the input selector board, but really, stick about 1V up it from a signal generator and work your way thru it with a 'scope, where the signal vanishes you have your problem.
What are you measuring 0.207mV with? That seems awful low for the output of a CD player, they usually swing a volt or so....
Regards, Dan.