Problems with LAMM LL2

I have been having a problem with what appears to be some type of distortion from my LAMM LL2 On occassion (not all the time) my speaker cones show large excusions in and out, even with nothing playing.

The reason I implicate the pre-amp is that if I take it out of the audio chain and feed my PS Audio DAC directly into the amplifier, everything is fine. I previously had a DC offset problem with this pre amp which was corrected by making two little boxes with a capacitor in each to put in line with the interconnects going to the amp. I don't think this is a DC offset issue because the cones go both in and out and it now doesn't matter whether I have the capacitor to filter out DC or not.

I sent the preamp into LAMM for repair and they replaced everything they could think of that might cause the problem and returned the pre to me, but the problem has persisted.

They said they tested it before returning but I can listen to the system for 10 or 15 hours before this happens, so I'm not sure they tested long enough.

Any suggestions on how I might diagnose this? I am on the verge of buying another pre-amp just to prove to Vladimir that the LAMM is the problem.