i never bothered, and will never bother, to see its sequel

Stonehenge! Where the demons dwell.

Phantom of the Paradise, probably. That's one of my absolute favorite movies in general. It tells the story of Faust in a more rock & roll manner, set in a rock club called The Paradise. It's not as much about being in a band, per say, but it does center around the music club, composer, owner, house bands, and selling your soul for rock and roll.

18"

I'd have to go with Almost Famous, although Detroit Rock City was a fun movie.

that's probably my fave De Palma

but as you've said, it's not exactly about being in a band

I didn't see the second and I don't plan to. Wouldn't want the first one to be ruined for me.