I'm thinking the possession thing.
Both channels are experiencing the volume change? While I have never heard of such a thing... if it is in one channel, I would suspect a tube. ARC is really well made... so the most likely culperate is a tube.
ARC will have one tube for both channels or two or more per channel. If the problem is in one channel. Then switch the 2 or four tubes per channel in the Amp first and see if the channel changes... then try in the Preamp. If it is both channels, get a replacement and se if it goes away.