Well, I'm jumping in here. I have a 210 Flex in my possession and am hoping it will be sufficient (with PA support, of course) for the Boulder Theater (capacity 1200) and the Fillmore in Denver (5000?). Haven't had a chance to really fire it up in a big room yet. Am I expecting too much from it? Head will be an Alembic F2B and Crest Powerlite 3.0. Excited but a bit nervous.
As with any cab, there are a lot of factors that go into whether it will be "enough" or not -- how loud you play, how much deep low end is in your signal, room acoustics, bodies in the room, the mix, so it's hard to give a definitive answer. Mike did mention building a modular 310 Flex in the past for a performer who ran insanely loud stage volumes.
That said, you've got PA support. I've played a 210 Flex (modular version) at a few loud shows with and without support (albeit smaller capacity than you mention) and never came close to maxing it out. It's efficient, voiced (with the switches it has lots of voices) to be present (without being obnoxious) in a busy mix, and will go LOUD. Just how loud, I don't know, but I think safely as loud as a really good 610. Loudness aside -- or maybe in addition to the loudness -- the lows and lows mids are incredibly clear -- my impression listening to mine while another player sat in was that it cleared up the sound of the whole band -- it retains its dynamics better at volume than just about anything you've heard, and the dispersion is great. Put another way, if it's a "one fridge" gig, I think you'd be okay. A "two fridge" gig, I don't know. That's my best guess, anyway.
I haven't played the Alembic pre -- if it has a lot of sub-lows and you're going to be pushing the rig, you might benefit from using an HPF with it, regardless of cab. The Prolite 3.0 is listed at 870 W/channel @ 4 Ohms. If this is legit, I'd think it would push the cab pretty well. If you do bridge, be careful with that 3 kW.
When are the gigs? If you have a chance to open it up with your setup beforehand, I would.
Good luck!
