So I've carefully thought out the power situation for that amplifier, and now I've decided to add a bluetooth whosamajiggie.
Time to crack it open and see how I'm gonna power it.
Fortunately, many people have cracked these things open already. They apparently suffer from a range limitation; the (metal) weight in the enclosure interferes with the bluetooth antenna. So folks have figured out how to remove the metal base to improve performance. Which, of course, I'll do now that I'm opening it up anyway.
On the inside is not a whole lot.
The plan, then, is to pull off that power socket; add a 5v linear regulator (this thing eats ~150mA, so that should be fine); put it back in the case, and wire it up to the amp. Something like this, perhaps:
Yeah, that'll do.