I went to McMasters training, and have a working acquaintence with the products.
Popping through your speakers is not good. The pre amp could have some bad capacitors. The issue here is that if something goes, you could conceivably pass dc, that is direct current, destroying your speakers.
Call McIntosh directly, and get their service department on the line. In the meantime, if you bought it from a local dealer, and it is relatively new, ask him for a brand new one to replace it. McIntosh is really, really, good about this kind of customer service.
Oops, another thought, though a little obtuse. could the popping be coming from noise in the line, and simply coincidental to turn on. (Refrigerators coming on etc send jolts through the line, and I have actually seen this blow tweeters) But that is not the issue since, it only happens on turn on. Just a random thought.