I appreciate your help. The 12ax7s are all measuring 5.8V.
I clipped some extra resistors in parallel with the power section cathode resistor/bypass capacitor, and it got noticeably louder.. probably at the expense of my tube life.. but man is this thing a joy to play through. I just need to put in a larger capacitor for the power supply, and this amp will probably be about finished.
Maybe your wall voltage is a little low--I expected the 372DX to give you at least 6.3Vac if not a little higher. Never had a Hammond that didn't give me the rated voltage.
Going back to the original sin of connecting the 5Vac center-tap to the power tubes cathode... I'm now concerned that you may have damaged some preamp tubes. The mistaken connection basically elevated your heater supply by your HT voltage. (It probably never got to 370V because the poor 5Y3 was struggling to conduct that much current and the cathode resistor was in the process of melting down.) There is a maximum heater-cathode voltage for the 12AX7 and it is 180V. For the EL84s, it's only 100V, but you were putting HT on the cathodes and heaters of the EL84's, so they were probably safe from arcing. Could there have been some arcing and damage between the heaters and cathodes? This is just theoretical, but might be worth exploration.