Weird crackling from speakers when preamp connected

I've swapped tubes & confirmed they're good.

Sorry, that sucks.  What maintenance did you have done?  The times I’ve heard something like that was from dirty connections between components and interconnects, so if you haven’t I’d try cleaning those connections especially on the VTL with Deoxit or something similar if you haven’t already.  Pray you don’t need repairs from VTL because their reputation for service, especially if you bought used, is awful.  Best of luck in finding a solution. 

I had it looked at by a professional, they ran tests & checked the tubes, no issues when they had it hooked up and couldn't reproduce the issue. I haven't tried to clean the terminals, but may give that a shot.

That has got to be frustrating. 

Perhaps it needs to go off to VTL.

I remember giving a tape deck to a professional electronics repair a long time ago and having it never sound good again. They can be good for replacing and capacitor or resister... but sometimes not for nuanced stuff. VTL, shoud  have an engineer that knows were something like that can happen and find and fix it. Hopefully 

I’d be going back to that service guy and politely ask WTF???  It was working before he got it and doesn’t work fine when you got it back.  Pretty damn simple math there, and I’d be right pissed off if I was you.