from the first your half of your words i suggest replacing all coupling capacitors immediately!
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There is a negative DC bias voltage applied to the grids of the tubes. If you lose this voltage the tubes will run wide open and blow the fuses. That is a likely explanation. A cold solder joint in the bias circuits could be the culprit or some part that is intermittent. These can take a while to track down and many times the people at the factory don't want to put in the time necessary to find it.
I think the mismatched tube theory is bunch of balogna. It might cause some tubes to run hotter than others but won't intermittently light them up and blow the fuse. This is the kind of crap you get from techs who can't find the problem and just want you to go away.
Not being familiar with the layout of your amp limits the ability to troubleshoot. Do you have a schematic?
When you say not the same set each time, do you you mean that sometimes it is the left channel and sometimes the right?
After it blows the fuse, can you just replace the fuse and it works again for a while?
Reading of your experience had greatly modified my opinion of Mr. Modjeski - maybe he can't or won't really try to do the repairs (bad enough), but he should at least be honest about it...this is just ridiculous. Since the amp apparently requires matched tube sets and manifests the problem one channel at a time, I guess it has auto-biasing that operates on a per-pair basis - this circuit would be high on the list of suspects to me...seems as if it can't deal with normal bias drift as the tubes age, or is prematurely wearing them out with incorrect bias. Anyway, you've proved it couldn't be the fault of the tubes...what a load!.
"Do you have a schematic?" Yes.
"When you say not the same set each time, do you you mean that sometimes it is the left channel and sometimes the right?" Yes. And sometimes the inside pair, sometimes the outside pair.
"Reading of your experience had greatly modified my opinion of Mr. Modjeski" I had a pretty high opinion of Roger myself, at least as a designer. Met him once at a CES show. My personal opinion was skewed a bit when I made an appointment to bring the amp to him to modify so that it would correctly bias the 6990s (the original circuitry -this is serial # 20 - slightly over-bias these). I drove 150 miles to his house in Santa Barbara and he wasn't home... No call, no note, no nothing. When the 6990s started acting up, his minion at RAM Labs at the time said to me "Well, if you're going three months without a problem, you should consider yourself lucky." That was the last direct dealing I've done with Music Reference/RAM Labs...
Forgot to answer this one:
"After it blows the fuse, can you just replace the fuse and it works again for a while?" Yes.
I just lived with it for awhile, but on this amp the output fuses are inside the case, so I have to dismantle the hookup, turn it over and remove the bottom cover to change 'em. Before I finally put it away I had considered installing fuse holders on the back of the case, but wasn't sure if I would introduce any noise by routing that signal off the PCB.