kts2001,
I decided to hold off for a sub in addition to the 624s for the same reason Lazi listed - I like the badumpa-dump-dump when I'm creating/mixing tracks. I've done only 1 session with 624s and a the mackie sub, and i was in heaven. The cool thing with the mackie 3 piece setup is there's a footswitch you can use to activate or bypass the sub. I'm no expert, but that setup worked well for me. I too record & mix all types of stuff with no problemo on my 624s. The 824s have great(er) bass response, but again, I want a sub in my room, and if I'm not mistaken, when the sub is engaged with the 824s, all the low-end glory the 824s are known for is shot through the sub, anyway. Same with the 624s, I know, but if that's the case, the 624s are plenty cool for handling what doesn't get diverted to the sub at the crossover point.
I think an equally important question is about sub-placement. As I've come to understand it, the "hard & fast" rules, aren't exactly so hard or fast, and I see a complete movement that believes in what is considered less-than-conventional principles re: sub-placement. Seems that if the rumble factor is that important to you, you'd be a good candidate for considering a sub set-up. Don't think, however, that 624s or 824s without a sub won't do you justice, they will.
I think the size of your room will demand that, especially with a sub, you'll need to be very careful with your acoustic treatment - bass baffles in particular. There are other people on this BBS who can better address this besides me.
I also think that the 624s will be plenty-plenty-plenty for your room even without a sub, and the 824s will thump in there. Ultimately, you can't go wrong with either setup. If you've heard both sets of monitors, then you know that the 624s translate quite well when it comes to low-end, and the 824s do it damn well, too.
I'd suggest that you post in the mixing/mastering forum (or studio building forum?!?) and ask some clear questions about speaker placement re: mixing with subs (or not). I'd use that more as a basis for making my decision as opposed to the "which sounds better" scenario. They can both sound good, and they can both sound not so good. They both have pros & cons. They each have a different set of demands/requirements, and they each offer something that the other may or may not. Do a bit of online research with some companies that sell satellite systems - suck up their PR about why their system w/sub is so great, and peel off the hype. You should find some clearcut similarities re: the "advantages" (or not) to using a system with a sub.
That's the best I can do without risking giving you some shaky info, and though I'm by no means ashamed of the size of my "tool," I don't wanna flash everyone and risk getting smashed by "the whopper."
(ah, the curse of "gear" envy, right?)
Good luck... Let us know what you decide & why...
Flo' Dolo