It's bad practice to leave grids floating (ie when the 'other' channel is selected);a Vdc reference is required, eg 1M to 0V, whenever plate current may flow.
The tube numbering is a bit wacky, there look to be 3 V1s! (12AX7, 12AY7 and a 6V6).
The signal from the pentode channel is being fed into the feedback loop (if engaged) inside the loop; that may lead to unexpected results, as it will be an error signal that loop will attempt to correct.
Gain wise, bear in mind that a 12AX7 LTP has a voltage gain of about 30 (= 30dB); so you're 30dB down on that, which is a big drop compared to a Route 66.