Baffling Tube Preamp Problem--Rhythmic Thumping after Tube Replacement

VTL Deluxe Preamp and VTL Compact 80W monoblock amps. I've had these for 30 years.

The two amps had tubes that were biased correctly, but there was a lot of static that I narrowed down to the preamp. The preamp has 5 tubes, four 12AX7 and one 12AT7. I tried replacing some of these with my spares but the noise persisted and the tubes were making noise when tapped.

I took the preamp into a local shop (the guy has tube amps and also has a vast tube collection), he tested/matched new tubes and installed them. He tested the preamp with a solid state amp and it worked fine. He used the "A" out puts which are designed for solid state. I took it home and plugged it back into my tube amps with the "B" outputs which are designed for tube amps.

The speakers on both channels started making a low rhythmic "thumping" noise, like a heartbeat, and then one of the amps blew a fuse.

I tested the tubes on both amps. The amp that blew was all messed up, but the other amp tested fine (at least the 4 power tubes 5881), I didn't test the two AT7's on there.

So I connected the preamp only to the amp that had OK tubes and the thumping problem persisted.

We are both baffled. It must have something to do with the B output on the preamp but the schematic is not particularly helpful and the company hasn't been communicating.

The thumping problem didn't occur until I brought it back from the shop, but maybe something came loose in transit.

Another possible issue is I may have put the wrong tube in a spot (i.e. AT where it needed AX) as my eyes are not great. But again, after he replaced everything, it worked fine with the solid state amp in the A output.

Any ideas on what to check next?