Well, after seeing one of these go on eBay for $600 (!), I decided that $400 isn't a bad price. So I bought it today, and it's just lovely.
It's a '73 or '74, and the crazy part is it still has the original Magnavox tubes! They're pretty hazy, but they work fine. I'm trying to decide whether or not to retube it with a quad of JJs from tubedepot. I'm not sure I should trust 33-year old tubes.
Anyway, thanks for the feedback, guys. I can already tell I'll be keeping this amp around for quite a while.