TU-8200 fault isolation guidance - left channel dropped

I have completed my build of the Elekit TU-8200 and am having an issue with the left channel. No sound and the left channel tubes are significantly cooler than the right. I did modify the build slightly by replacing the standard volume POT with a TKD 2CP-601 POT and by replacing the 8200 transformers with LL2777B transformers (the 8200VK option).

The schematic for the 8200 is here:

Initially, the left channel would work fine for 5 to 10 minutes and then drop out. I did some basic research and troubleshooting (swapping tubes around, checking cables and connections…) and made an assumption that the Q1 transistor, part of the active automatic bias circuit, was bad and replaced it with another A1266 transistor. Since this replacement I have had no sound on the left channel after powering the amp. I have done some basic troubleshooting since then, visual, continuity, hot spots and I am not seeing anything obvious to my limited experience. I’m at a loss and am concerned that my limited troubleshooting ability has made things worse.

Below are my readings at the test points listed in the schematic:

Test pointMeasurementTest PointMeasurementExpected measurement
1.37 VDC23.8 VDC3.2 - 3.7 V
3.26 VDC43.7 VDC3.2 - 3.7 V
512 VDC6118 VDC100 - 120 V
715 VDC8117 VDC100 - 120 V
922 VDC10119 VDC195 - 220 V
111.1 VDC121.1 VDC1.0 - 1.2 V
13-9.1 VDC14-19 VDC-12 - -24 V
1575 VDC16300 VDC265 - 290 V
170 VDC18.44 VDC0.4 - 0.55 V
1934.0 VDC20283 VDC255 - 280 V
2133.8 VDC22276 VDC250 - 275 V
2333.8 VDC24289 VDC255 - 280 V
25304 VDC 270 - 300 V
26-32 VDC -29 - -35 V
27-34 VDC -30 - -36 V
286.3 VDC 5.6 - 6.4 V
296.6 VDC 6.0 - 6.8 V

Any guidance and advice is greatly welcomed.

FYI I have cross posted this in r/AskElectronics.