MAke sure the speaker is plugged into the MAIN speaker jack, not the extension speaker jack. The MAIN jack is the one with the shorting contact across it. The shunt opens when a plug goes in, but if you use the extension jack, the shunt remains.
Testing plate resistors couldn't be simpler. Go to pins 1 and 6 of the 12AX7 tubes. If there is B+ there, the resistors are not open.
You have resoldered the small tube sockets, but the power tube sockets are the ones that crack their solder most.
Circuits are circuits, so it doesn't really matter what the parts are on, turrets, eyelets, terminal strips, pc boards.
Use the FX loop to test. Plug the guitar into the FX return. Is the output strong or still real weak? And plug into the regular input jack and connect the FX send to some other amp. This tests your preamp. Does your preamp sound OK coming out that other amp? We just determined if the problem was preamp or power amp.
I use my eyes for a quick check, but if I suspect a solder joint, I just resolder it. Why waste time trying to see a crack that may never be visible? My iron is already hot, and resoldering a tube socket takes literally just a few seconds, lots faster than me and my hand lens.
In fact that whole Fender Hot Rod series and their other amps made the same way, like yours, I just automatically resolder the power tube socket pins.