[MrFixr55]

If you sub diodes for the 5X4, then yes, the variac will allow a controlled buildup of B+. There is a whole theory of cathode stripping when B+ comes on strong immediately, but most rectifiers like 5X4s, 5Y3s etc. come up before the heaters of tubes like 6K7, 6F6, etc. come up so I would not worry about it.

However, you raise 2 points that tell me that your issue is not B+ or audio:
1. That in peaking, you had too totally crank down on a trimmer cap and volume increased but did not pass through a peak.
2. That the AVC at the IF tube grid was only -2.5V. This tells me that the issue is in the "radio" section and not the Amp or PS section.

Sorry, I have not posted detailed instructions for the "aux jack" I will when I get a chance. Do perform this mod as the amp section of
this radio makes a great tube amp, guitar amp, etc. Even without doing the mod, you can inject a signal into the pot at the ungrounded end. Tune the radio to an "dead spot" where no signal is received and connect an audio source such as an mp3 player to the ungrounded leg of the pot (C61 connects to this point) and ground. This will put the Audio and Power supply issues to bed permanently.

When you aligned the IF, what was your sig gen set to? is it accurate? IF for this set is 470, so if you set for 455, this may be an issue. Set the IF for 470, then ensure that the oscillator is 470KHz above the signal.

AVC works by taking a voltage from the detector stage and filtering it to a pure negative DC. The stronger the signal, the more negative the voltage is generated. This is applied as grid bias to the IF tube and possibly the convertor and RF tube. The more negative the grid bias, the less the gain from the stage. This AVC voltage is also used on tuning eyes, shadow meters, etc. So the AVC of only -2.5V indicates a weak gain in the radio section. Work this area first.

Hope this helps.