Just for reference to anyone else who has this problem in the future, I was able to get -6dB attenuators from Parts Express that did the trick nicely.
The "pop" is still there - but it's so much quieter you can barely hear it anymore.
Oddly enough, there's a new sound that's appeared since adding the attenuator - any time the Marantz receives an IR burst - even if the IR is intended for another component - I get a wierd beeping from the same troublesome right surround channel. It sounds sort of like a modem or a fax machine, and is only a second or two long.
Clearly the preamp has issues, but it should get me by until I can afford an Acurus Act-3 or an Outlaw 950. Considering I paid 70 bucks for it on Ebay, I don't really have any right to complain!
Thanks for the suggestion, Sean; no luck however. Our power does seem very dirty and unstable - it's an old house with old wiring in a dense urban neighborhood, and I'm sure that doesn't help. If I could afford a nice power conditioner, I'd get one, but AC stuff is just too expensive. Especially having the quintet of MA-500's - that's 5 outlets right there!
Thanks to everyone for their help in this thread, and over email.