ADCOM GFA 555-II distortion alert?

After running an hour or two, rt. "distortion alert" indicator lights on my 555-II. For the most part, this does not result in a readily audible difference, although I have gotten fuzz on the right channel. Local service center could find no problem; said it must be a problem elsewhere in my system. I've run it with two CDPs (NAD, Sony), two sets of speakers (DYI, Onix), and both stock/zip and Signal Cable interconnects/speaker wire. Only the ADCOM GTP-400 pre has been constant; could it be the pre, or is it more likely, as I think, the amp? Any thoughts much appreciated. May be time to replace, but it has been a good amp. THX! John