I'm inclined to think that the culprit could be something with the preamp or the new tubes.
When you make a change and things go south its a good bet that the problem is associated with something you did.
However, that bet can also blind you to what is really going on; pursuit of the problem in that manner can yield no results if its not really the issue.
Equipment turning on or off can create pops that come into the equipment on the AC line. IME, most so-called high end audio 'conditioners' are not really all that good at their job and may be unable to filter such events, or they could even cause them. Some preamps are more susceptible to this sort of thing so it is possible that the Cary is working fine but simply can't reject events like this. I see this hypothesis as unlikely since we'd have heard of it before on this forum...
And of course the popping could be a failing power tube that is arcing.
So it is worthwhile to do your due diligence.