You fellows have been extremely patient and helpful. Thank you.
Tonight I have determined that my Adcom GFA-555II power amplifier has indeed failed, apparently as a result of a transient that came through the PS Audio phono preamp unit. The amp powers up with its "thermal protection" LED lit and the left channel is dark. (Trust me, I've pumped some current through this amp for years and have never gotten it warm enough to thermal out, so something has gone "fizzle" in there.) I will attempt to evaluate and repair— I like this amp.
As for the AT120E phono cartridge, I have yet to figure out if it's a moving coil or a moving magnet, though I found a comment on an audio forum that said it was a M-M. I had poor results with both switch positions on the PS Audio unit (before I blew the amp) and a careful inspection of the preamp's internals shows no modifications or burn marks. I suspect it was just a really old, dirty switch that somehow produced a nasty transient. Damn.
So I resurrected the old Carver Receiver and am using its power amp section to drive the big Acoustat 2+2s. (AIFF > iMac > ARCAM rDAC > PS Audio IV > Carver.) I think it sounds thin compared to the Adcom unit, but at least I have tunes.
The irony here is that the Carver's phono preamp works great— it always has. I still don't know what's the deal with the PS Audio unit's phono preamp, but I might pay someone to evaluate it, as long as the poor Adcom power amp needs a service call anyway. The PS Audio unit should sound good if it's working correctly.
So there's my sad tale. I'm really not as hapless as I sound— I carried a tool bag as a field service engineer for many years, back in the day when FSEs dragged oscilloscopes through airports and would perform component-level repaitrs on proprietary scientific instrumentation. Wish me luck, and PLAY IT LOUD!