Any electronics wizards? Tube problem with RM-9

I have an early Music Refernce RM-9 Mk.I amplifier that sounds absolutely wonderful - when it works.

I bought it used about 8 or 9 years ago and have had a reoccurring problem with it since day one: after some time of operation (days/weeks/months) one pair of output tubes (not necessarily the same set each time) will go into saturation, lighting up and eventually blowing the output fuse.

I sent it back to Modjeski twice and they kept telling me it was mismatched output tubes, and techs at two other shops couldn't find any problem with the electronics of the amp. I went through a multitude of "matched" tubes from a variety of sources and eventually ran into the same problem with each new set.

The tech at Music Reference suggested I was pushing the EL-34s too hard and suggested switching to 6990s, so I bought a set of RAM 6990s and they went about 3 months before starting to do the same thing.

The amp has been sitting in my closet for about 3 years now, and since I just purchased a pair of ProAc Response 2.5s I'd like to bring it out of hibernation.

The only outside influence I can think of which may contribute to the problem is I've always run the output through a Stax SRD-7Pro box (this amp is really sweet thru 'phones), but other amps (tube and SS) haven't had a problem with this load at all.

Why are my output tubes saturating???