fsonic, I don’t know what I said to make you so angry, but you DID write, ..."or degauss the current mat, or put a blocking capacitor somewhere." And this is what led me to believe that you were supposing that a capacitor can block a magnetic field. Even Sherlock Holmes could testify to that fact.
Mitchell, I apologize if you felt that I was discounting the thoroughness of your analysis. I was not, although your report as it is does suggest a few other experiments that could be done to investigate the matter further, rather than for the rest of us to just keep on guessing. The first of those would be to re-install the XLR connectors and determine whether you can reproduce the original phenomenon. But I also understand what a pain it is to change the connectors on your ICs, and if you don’t want to be bothered, that’s fine.
My point was and STILL is that it could be that some piece of gear in your signal chain (originally I mentioned the preamplifier, but it could also be the amplifier) is induced to oscillate under certain conditions. And this may have nothing whatever to do with the platter mat plus or minus an LP on top of it. (Hence, I used the term "red herring" in reference to the bit about the effect of the platter mat/LP) Oscillation can be very capricious, intermittent, and nearly impossible to reproduce when you want to investigate its cause. Turning a unit off will stop its oscillation, and when you apply power again, the unit may work fine under the exact same input and output loads. Like I mentioned, I have had a personal experience with an unstable amplifier that vexed very competent repair persons and then vexed me for another year when I took on the job of fixing it because no one else wanted to touch it. This amplifier oscillated when fed by a certain CDP but not when fed by another CDP, for example. On other occasions, the amplifier worked fine for months before it would go into oscillation solely due to music input. From a pragmatic point of view, if you are happy with SE operation, and if your particular problem never recurs, you really never have to know what was going on. Just enjoy the music.