ARC LS25 MK1 Trouble-Shooting

I would appreciate any comments on trouble-shooting the following problem. I moved my Audio Research LS25 MK1 pre-amp to a new audio rack and re-started my system. Initially everything sounded fine for a few minutes until I heard a "pop" sound from one speaker with the source material (tuner) still being amplified. I changed my source from tuner to CD and there was initially no problem. After about one minute the only sound was a hissing from the one channel and then my power amp shut down. I replaced the pre-amp function with that from a receiver and everything operates correctly. As for some history on the ARC LS25, I purchased it used here on Audiogon about a year ago and have had no problems. I also powered up the ARC pre-amp and verified that all four vacuum tubes were glowing. Is there anything else that I should check before calling Audio Research on Monday? Thanks for your help.