Thanks Coco and sassafraz.
Fixed it! Amp is working fine again. Forgetting to take the tubes out when measuring those voltages was a big brain freeze.
Went at it again last night. Before doing any more measuring (per Coco and sassafraz's directions), I went ahead and swapped a few more tubes just to be sure - including a brand new rectifier tube. Nothing changed. That meant more measuring, and this time the right way!
This time, I began by taking out v1 and v2 (for a start) and measuring voltages and continuity with my meter. Immediately, things looked different. (The light bulb above my head began coming on!) As expected, I got the 3.15 or so volts from pin 9 to ground and from pins 4/5 to ground on v2. I also got the 6.3 volts between pins4/5 and pin 9 on v2. Nice, everything looked good there. A nice baseliine, if you will
But once I moved over on v1, I only got that voltage on pin 9, not on pin 5/6. Ruh-roh!
Of course, I also didn't get the 6.3 volts across the heater wire pins (as I did on v2). Something was clearly up!
I flipped the chassis over and measured all the voltages again, but this time from the tube side (all others were from inside the chassis). I poked my gage probes right into the little holes. Sure enough, I got 3.15 volts on pin 9 of v1, but nothing on pins 4 and 5. As I checked v2 again, things got more interesting. I got the 3.15 volts on pin 9 and pin 4, but not on pin 5. What? That was weird. Double and triple checked it. I did not get the proper heater voltage on pin 5. And obviously, I wasn't getting it on pins 4 or 5 on v1 (which is fed by the wire from v2).
Upon further inspection, it looks like my solder joint that connected the following items wasn't very good:
1) connection of pins 4 and 5 of v2 to...
2) heater wire coming from pins 4/5 of v3 and ...
3) heater wire going to pins 4 1nd 5 of v1
The voltage was arriving at v2 from v3, but wasn't getting sent to v1. It was only showing itself on one of the two joined (I push them together) pins 4 and 5 at that tube socket. And given the orientation of the jumble of wires at those two pins, it wasn't getting to the one wire that left to supply v1.
This is crazy since the amp actually worked perfectly for months. I either had a cold solder joint, a really weak solder joint, or (I embarassed to say) a joint that I totally forgot to solder. Yikes! Must've simply shook free. Looking at it as best I can (given my 44 year old eyes that aren't what they used to be), I can't rightly say that I soldered that joint. Amazing, since when I look at the entire chassis, everything else looks fine.
I simply heated up the gun and re-flowed (or just flowed) that one connection. Bingo! All voltage readings now look fine and the amp plays like a dream once again.
You couldn't have wiped the (albeit sheepish) grin off my face if you tried.
Thanks to everyone that provided me with some direction, especially sassafraz for the reminder to take the darn tubes out before measuring those voltages. D'oh!
Now that it works again, I may need to start looking into VRM...