Not that all high end cables are bad (this forum seems to be plagued with cable haters) .. but I would certainly look into that particular cable as possibly being the culprit, especially if no one could ever find any problems with the amp. Those little boxes on the cable are just stuffed with useless electronics that could go bad and cause problems with the amps.
Your friend needs deep therapy. He has obviously been fleeced for years by a high end "Audiophool" dealer. Its a good thing he has now closed his doors.
Having amps repaired 14 times is insanity, as repeatedly doing the same thing and expecting a different result is diagnostic.
What is causing the problem is difficult as this system is awash with culprits.
Lets begin with the tube amp. Acoustic Research of Annapolis Lane Plymouth Minnesota was founded by William Z Johnson in 1970. They specialize in massive vacuum tube amps among other tube products. They are now part of the McIntosh Group.
If you must have a tube amp these are among the best of them and they are an ethical company even if selling monstrously expensive tube amps, which are arguably inferior to good solid state amps. This is especially true driving difficult speaker loads as they have output transformers which don't perform well with varying impedance loads and negative phase angles between voltage and current. So if you do insist on a tube amp you have to choose your speakers with great care.
That brings me to Watt speakers. Watt are well up my list of lousy speakers to be avoided despite their high cost. I have never heard one of their speakers I like and I strongly suspect the speakers are incompetently designed.
In reference to the problem at hand, these speakers are the most likely cause of your friend's amp failures. So let us look at the impedance curve and phase angles.
There are two things that strike me.
The first is that the impedance actually drops to almost 2.0 ohms where there is a lot of power required at 250 Hz. Now I would be strongly suspicious that that reading is lower then the DC resistance of the drivers at that frequency. If that is so then the crossover is in resonance. That alone would explain your friend's difficulty.
But there is another serious problem.the phase angle is going off the bottom of the chart at 6 KHz. It is -90 degrees and still heading south. This means there is a huge gap between voltage and current phase. Now this is just the sort of thing well known to send tube amplifiers in to ultrasonic oscillation. The oscillation is well above the range of human hearing. This problem really blows up tube amps.
So we already now have two causes that could be responsible for these disasters. You absolutely have no business marketing a speaker with curves like that.
In fairness to Watt, this sort of thing is prevalent in the high end exotic speaker market. This was addressed by Dr Floyd Toole in these forums in the recent past. He specifically called them "incompetently designed speakers requiring arc welder amps!" This does not excuse Watt though.
Exotic powerful speakers are not well served by passive crossovers and are much better served by electronic crossovers with multiple power amps. I use three amps per speaker in my reference design.
Lastly the cable. This could also have a bearing on the issue. The marketing hype surrounding exotic cables is made up out of the back of the neck and or delusional. As Peter Walker of Quad used to say, "Bloody wire is just Bloody wire!" And he was and is still right. These cables costing thousands have no advantage over zip cord. Worse quite a few exotic cables have high capacitance. If his MIT cables do, then that would increase the potential for ultra sonic oscillation.
Now I'm particularly concerned about these cables. The designer used to do work for Monster Cable, one of the most unethical companies around.
This is what I'm particularly concerned about. "The termination housing features a three-position switch marked SD, HD, and SHD. Two six-position knobs on one end of the termination box are marked “Articulation,” one for the bass and one for the treble. looking next at the interconnect, its termination box is small in comparison (3.5" x 6"). It offers two adjustments, one to match the cable to the input impedance of the component it is driving, and the other a five-position knob marked “Articulation.”
This sets a new low or high in Audiophoolery whichever way you want to look at it.
In summary, you friend should definitely ditch those cables. However there is a high chance he will still have the problem. He does really have to decide whether he wants to get rid of those speakers and get ones more compatible with tube amplification, or get rid of the amps and find an "arc welder" category solid state amp that will drive those speakers.
This case I think is the worst we have seen of the damage wrought on people by the lunatic high end. This is not innocent marketing buyer beware but I believe a deliberate effort to defraud which is criminal.
