[kalee20]

Reading down, I'd have said, Ah! Residual IF being amplified by the AF amplifier and getting radiated by the wiring back to the IF amplifier input! But, that seems to have been covered.

I'd also have said, Bad chassis connection somewhere, putting a common impedance to true ground in a high-level stage and a low-level stage! But that seems to have been covered too.

When you tried extra capacitors stuck on here, there, and everywhere, did you take the earthy lead to chassis, or to an existing, possibly dodgy, solder tag which looked as though it was a solid chassis connection but may not have been?

All I can think of otherwise, is, without the EL41 in, does it still oscillate? Of course, how would you know... But maybe you could monitor the AVC line with the EL41 in, see if it goes more negative as you approach howling, remove the EL41 and see if the same happens. It's just barely possible that IF is present on the AVC line, and winding up the volume control is affecting this, with the EL41 just being a red herring!