I gave this opinion in the [
="http://www.sweetwater.com/forum/showthread.php?threadid=5575"]Sweetwater Control Room Forum[/
] just yesterday...glad I could reuse it.
I'll agree that you really need a sub to mix with 624s. I have an el-cheapo sub that I keep trying to take out of my studio (ugly, noisy, takes up space), but no matter how much I cut the bass in anticipation of the 624s underreporting the bass, I get mixes that don't translate--way too much bass. Then I cut, and I cut, and I cut some more until I'mmixing with my eyes, not my ears, and the results are what you'd expect: random. However, with a sub, it's possible to hear what the bass and kick are really doing, and it's possible to make mixes that translate.
Mackie is not kidding when they put an icon for "loves the 12" sub" next to the icons for "fragile", "this end up", and "do not immerse in water". Let me reinforce that: you you plan to drop your 624s on a regular basis or leave them soaking in the bathtub overnight (perhaps after a good drop), then you're probably the kind of guy that doesn't need a sub alongside them. Otherwise, you do.