Actually, the jellyroll pan was a pretty good idea. I grabbed a heavy duty 9x13" one, and if I hold it over the Crack like an umbrella (tilted just a bit toward the west south west) it seems to block the chirping almost entirely! I can only assume this means the signal is coming from the mothership locked in geosynchronous orbit somewhere up in that vicinity, but I'm much more interested in enjoying my amp again than in deciphering whatever message they're trying to send me
I'm going to try wrapping the base in foil too, like you suggest Derek. If that works just as well, the copper film tape will be an elegant solution. If not, gonna buy a few more jellyroll pans and break out the hacksaw to build an enclosure for the amp to live in.
Really appreciate all of the help and suggestions in tracking this thing down (and more importantly, ideas for how to block it)! Can't tell you how much the chirps have been driving me mad!