Tube question for the electrical engineers

A few years ago I decided to try a tube preamp to pair with a SS power amp. That choice became a McIntosh C220 which now fronts a Mac MC-402 ss conncected through a Mac MEN 220 unit (room correction).

My only ongoing disappointment about the C-220 is that the tubes in the phono-stage (2-12AX7) seem to be extremely vulnerable to wear/burnout, even (for want of a better word) blowout. I've been told more than once that I can expect "up to" 10K hours of life out of these tubes (Mullards) but I've never been close to that. I put new tubes in and they're dead quiet for awhile, even with the gain all the way up, then after awhile--a couple or few months--I start getting noise in them. To be sure there have been some extenuating circumstances more than once. The first time there was a power outage and when the power came back on the tubes blew--my recollection is that it was at least the linestage, if not both line and phono. So I invested a couple hundred bucks in a Brickwall unit.

That seems to have saved me from "total" blowouts but I'm still getting what must be abnormal tube wear. I detected background noise twice in the last year+ and replaced tubes (the first time all of them, the second just the phono stage). Everything was fine, or seemed fine, until the other day when we had a micro-second flicker in the house power which was not enough to reset the various digital clocks in the house. At that time, and I had music playing, the MEN 220 shut down but the amp and preamp did not. (I should add that the amp and preamp are ganged together with a little cord of a sort that allows both units to power on when one "on" button is pushed. The MEN220 is not so ganged at this point.)

When I powered the MEN220 back up I flipped over to phono (no turntable running) and ran the gain up and there was noise. It wasn't so loud as to noticeably intrude on music at a reasonable listening level (and so I cannot be sure it wasn't there to begin with before the flicker) but it was, and is, there. So the question is, aside from the power anomolies described, will tubes wear out in such a way as they are still amplifying but are making progressively more noise? It's rather irritating and it's costing money. Should I contact McIntosh? Should I use a power conditioner instead of a Brickwall unit? Help!