Sixty Five 112 Amp Repair

I recently purchased a broken Sixty Five 112 as a project amp for very cheap. I'm quite new to the world of amp repair, but have been doing reading, research, as well as some lower level projects on other amps and have been having a lot of fun with it. Fixing this Music Man amp felt like a great opportunity to really get into something and learn the ins and outs of an amp, which I still believe but I'm definitely in over my head at the moment.

The amp I have is chassis 2275-65 and is the older version with the 12Ax7 PI. The person I purchased it from bought it from a thrift/vintage store that had it labelled as "wired for 6L6s; needs transformer," which I don't want to just assume is accurate, because who knows who wrote that at this thrift store five years ago. It turns on and makes sound, but it has very low volume and sounds muddy and distorted at all volumes. I know this model was originally wired for EL34s and looking at the schematic for 2275-65, I think I've at least confirmed that somebody has modified it due to the addition of a 700 ohm resistor as well as the 1500 ohm resistor on the screen grid (pin 4) of V1, but I'm pretty fresh at reading schematics and the schematic I can find for the 12Ax7 version of 2275-65 is very difficult to read, so I'm not 100% sure what's all in there that's not original. I've confirmed that Pin 3 on both tubes are getting the high voltage one would expect in this amp.

My goal is to restore it to the original circuit and tube specification if possible, but ultimately I just hope to get it to be a usable amp. I plan to swap all electrolytic capacitors, but otherwise am not totally sure where to go from where I'm at. I guess my first questions would be

1. What is the best way to test whether the output transformer is functioning?
2. What other voltage readings should I take and where do I take them to start identifying other issues?