I see 801 or 802 in "every" mastering facility.

Why mixing engineers don't use these?

Most studios don't have the space for them, and the rest don't want them in the budget.

Some of the... how do I put this... "Money is no object" places (Abbey Road, Skywalker Ranch, etc.) use B&W 802's as mains. But they have the space and the budget. Including the budget to replace them from being abused during some sessions.

The problem is that you can't just plop them down behind a 96-input SSL. They're amazing boxes to work with, but they're also very picky about placement and amplification. When you're using speakers like that, you generally build the room around the speakers. Or at least build the room with a particularly sized floor-stander in mind.

That, and many places don't want to tie up $20,000 in a monitoring chain when they can get a good pair of powered midfields for $2,000 and then buy a bunch of microphones.

Thanks, makes sense.

For how long have they been around?
What kind of power amps are being used with these things?

They started getting studio use in the mid to late 70's if I recall. The Nautilus & "D" designs are fairly new -- Mid 90's-ish.

Amps... Pass Labs is a popular choice (Katz, Blackwood, me, plenty of others), some prefer Krell or Levinson, there are others...

B&W normally suggest Rotel... But they also *own* Rotel if I'm not mistaken (not that Rotel doesn't make a reasonably bad-a$$ amp).

Very good sounding speakers, big and refined...Levinson, Classe and Bryston are common amps with the 800 series too. You'll want good cables and clean power for a high end setup like that and a proper room goes without saying, I agree there.

The latest and greatest new 800 series are expensive as all hell but the line of speakers has been around for a while in various marks and sometimes you can find older used ones surprisingly (relatively) affordably, but most of my experience is with the more recent gens.