Do you use all NOS stuff? I have some old bastards kicking around from a couple ancient organs my friend and I cannibalized years back but most of 'em are too far gone, particularly for trying in like V1 and V2 in the DR. Too bad too because there are a few GEs in that batch. But as far as new glass is concerned, I've had good luck so far with Mesa 12AX7s and 12AT7s. They aren't TOO pricey per unit though a slight premium from like JJs and EHX.
My amp is really sensitive to what it gets in V1 and V2. I'll stick a tube that is fine further down the line up to V2 and it'll struggle. I'm not familiar with the Selmer's circuit but I bet it's a similar situation. You aren't red plating or anything, right? I figure your dude would see that right away. I learned somewhat recently that with these combo amps you can literally vibrate a tube to death so maybe that could be it, too.