Have you had the tubes tested? You may simply have a bad set of tubes in one amp...
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Try swapping the tubes between the two monos, this way you can determine if the tubes are to blame. No one likes to have a problem with their system, but what's even more frustrating is the inability to diagnosis the problem. In your case, I hope it's the tubes because that is a problem easily remedied without sending the amps to AR.
Upon further reflection, I've realized its a severe and irremediable problem. The only solution is to sell the remaining good mono amp at a heavy discount. By the way, have I told you I'm looking for an unmatched ARC mono tube amp for my center channel? ;)
You might try a visual inspection of the tubes before sending things off or swapping things around too much. When the KT88s blew on my old mono amps, it was pretty obvious. Fire the amp up with the top off and see if each of the tubes glows consistent with the others. Remember, its not just the 6550Cs that could be at fault--there are also two (? maybe more, there are two in each of my VT100s) funny little 6H30 tubes, as well. The hard part will be taking the top off. My recollection is that its about 90 philips head screws to remove that sucker.