Problem solved! I found the short I'd been looking for during a session of troubleshooting earlier this week.
After scrutinizing every solder joint that I had made I felt confident and tried another round of resistance checks. Again, this went alright, the same results as before. Then I loaded in the tubes and fired up the amp and no luck, still had no voltage across A1 & A4.
Scratching my head, I shut off the amp and checked the resistance of the path to ground from the tube heater circuit, with the tubes still loaded this time. And there it was, I measured between the center tap of the hum pot and ground and found 12.5 ohms, not the 1.2K like the B side.
I still didn't know where the short was but I knew it only existed when the tubes were in place. But I had already swapped the 2A3s as previously suggested and that didn't clear it up. That left me with the 12AT7, I removed it and that cleared the short.
At first I thought I might have bad tube, but I checked for shorts between the pins and it seemed okay as far as I could tell. I started tracing out the wires to the 9-pin tube socket and then I found it. Not what I had expected.
The short was on the power transformer, terminals 10 & 11, on the winding side of the lugs, not the solder joints that I had been inspecting. Both winding ends were bent out a bit and bridged with solder. I had to snip it and bend them back. Terminals 10 and 11 are the return side of 12AT7 heater circuit and the line side of 2A3 circuit.
After that I ran through the voltage checks quickly and they were right on...
Thanks again for your responses and input, every suggestion helped me to get a little closer
