The 624 monitor is physically smaller than the 824 and clearly has less low end. A subwoofer would be a necessity with the 624, whereas the 824 is fine without. The 624 has a front panel power button with LED, and also adds an overload LED. The back panel sports a series of switches customizing the monitor to the room, as does the 824. The 624 has a low frequency switch with 49Hz, and 80Hz roll-off settings.
The main 6.7 inch speaker is powered by a 100 watt FR series amp, and the tweeter is 40 watts. The unit has XLR, 1/4", and RCA inputs, accessed the same way as the 824, vertically at the bottom rear of the unit.
Overall, I liked the sound of this speaker. while not as full or powerful as what I'm used to with my 824's, the 624's have good imaging and sound clean. My guess is that Mackie is intending them for home/project studios and also as satellite speakers for surround systems. These speakers need a subwoofer, but with that would be very useful for mixing in surround in mid to smaller sized studios or post production rooms.
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