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.