Troublesome Audio Research DSi200

For any of you who have owned, or may still own this integrated amp, have you experienced issues with it? I bought mine used last year to supplant a Hegel H200 that was a little too bright for my taste and sucked rather a lot of electricity out of the wall even when in standby. In general I have liked the sound of the DSi200 but it does present a very slight hum through the speakers, a bit louder in the left channel than the right. After having it serviced TWICE, once by ARC and once by an different authorized service provider to address this issue, I am told that it's an artifact of class D amplification (high frequency switching) and so I guess that's just what this amp does. It's a soft hum and you really have to have your ear right up to the speaker to hear it.

Since it came back from its last service, it has been mostly sitting in its box (I'm currently using a Modwright KWI200) but today I took it out to do a little comparative listening because at some point I have to decide which of these amps to keep and which to sell. I thought that letting it run for a good week or so would insure that it's in full voice, but a few hours in, it began to buzz very loudly--the amplifier, not the speakers. Since I'm not an electrical engineer (far from it!) I really don't know what is causing this. I'd hate to go the trouble and expense of having it serviced a third time in less than a year, but obviously I can't sell it in this condition. Any theories about this? Have any of you had this happen? I'd be very grateful to hear your experiences. Thanks.