Gary you remember my comment in my last email?
O yes a subwoofer does not need to be connected to both channels one is perfect.
Can you measure the resistance between both signal inputs on your cable to your subwoofer. So not between signal and ground but between the signal wire which was connected to the left and right of your pre amp. This could be an important help in finding the issue. (I should have said here perhaps the reason of the issue)
Well let me explain here a experience I had with using cable making a mono signal from a stereo signal.
I listen in Mono so I connected the out puts of my grounded grid together to go to my mono power amp. (similar to yours how you connect the subwoofer)
When I had this configuration the left channel tries to drive the right channel and vice versa I learned, the lower the output impedance is the worse it gets since the current will be higher. It damaged for example my output tubes in the Grounded Grid over time. ( I noticed music was missing and one channel died)
I was overloadding my output tubes.
Now fast forward.
When I put a resistor from 1K5 in the signal path of both channels before they were joint to one output, all problems were solved. Now the resistance between the left and right is 3000 Ohms. And that is way higher then the output resistance from the output tube.
So again check the resistance between your two signal wires from your subwoofer. It should not be zero. If so make the modification, it could be that this had effect on your malfunction.
You can check if the LM317 is still correct by inserting the tube you know works. You can measure between the + side on the output capacitors and the ground, should be around 9 to 10 Volt.