I am happy to hear you found the problem with amp. I had this happened twice with my Mark Levinson 23.5. First time, it was a capacitor gone bad. So I sent the unit to George Meyers AV and they completely replaced all electrolyte capacitors. Then earlier this year, my left channel became barely audible. I sent the unit back to George Meyers and they found the problem and fixed it at no cost to me except shipping & handling.
So I had to do this drill to isolate the problem. I have tubes upstream and some tubes can give similar problems you described. I always start from the speaker side and work up the chain to all the way to source. One time, I had a bad tube in my Audio Research PH5 phone and right channel began to fad in and out.
You have an excellent system. But have you noticed that impedance of your speaker drops to 3.5 Ohms and your amp is rated at 160 W Class A at 8 Ohms? Since it is an all Gryphon system, it may be working. However, I would recommend you find an amp also rated at 4 ohm and 2 ohm preferably power doubles as impedance is halved. Since you are now looking for an amp, this may be an opportunity to find out if you can breath new life to your speakers. For an example, ML 332 advertised below may be a worth a shot. This unit was recently serviced by George Meyers.