Weird crackling from speakers when preamp connected

The service person and I talked extensively about the preamp, he ran tests on the device, tested tubes, no parts were replaced. They also hooked it up to a system and couldn't reproduce the issue.

Again it doesn't happen when I run my DAC directly to the amp, only when the pre is in the chain.

Are you using the RCA terminals on the Vega and the 275? (Just to confirm you are checking the same inputs and the XLR isn't bypassing an RCA input connection problem at the amp).

Crackling can be a bad connection on the RCA ground. But if the problem only occurs after warm up the first thing I check is the soldering at the tube pins. Then I check the volume pot, and the plate and cathode resistors by putting pressure on them with a chop stick. Things move when heated and a bad solder joint can lose contact, especially if the volume pot and sockets are circuit board mounted. If I was looking at it and could not reproduce the problem I would at least reflow all those solder joints.

This could be a factory lemon or it was dropped in transit. Ask the tech to take another (careful) look.

@gs5556 I’m using only RCA connections and not XLR. I’ll try your suggestions and have them look again. 

How old is the MC275?  I had a similar issue with a pair of Ohm speakers and a Sansui receiver that a qualified technician could not replicate in his shop.  Turned out to be the receiver.  It was 30+ years old and needed to be recapped.

failing capacitors (electrolytic) or tubes