I determined it was not the tubes, substituting known good tubes.

I plugged straight into the "power in" and determined the problem was in the power amp.

I replaced the plate load at R57 (82K), which is known to come open on Hot Rods, and R58 (100K) as per instructions at the "unofficial Hot Rod Deluxe" website. While I was at it, I went ahead and replaced the 470ohm screen resistors with 1K 3watters to 'improve reliability'. Everything else looked fine. I checked all my solder joints for continuity. No problems that I could see.

The amp works now but breaks up with single coils as early as 1.5 on the clean volume. That big, clean 9th chord "gank!" sound is gone. I checked it at various bias levels, it doesn't have the random channel switching problem some HRDs have, and it still goes fairly loud. But something is definitely wrong.

Thanks.