If you send to the BD1200-D first, you get the benefit of its built-in high-pass filter. That means that the sub handles the low frequencies, and they don't get sent to the SSv3. Since the SSv3 has a modest 8" driver, it has to work extra hard on those lower frequencies.
The net result is that you definitely get more volume and less potential for distortion. It's noticeable in most gig situations, especially if you're doing bass lines, etc. A nice upgrade for $299, plus cables.
(the never ending SSv3 thread)
And the Behringer sends L&R pre-crossover to FOH.
Wait, that's potentially game-changing for me! Let me make sure I have this. Are you saying that the THRU OUTs on the B1200D can be sent to the FOH carrying the full (pre-crossover) signal, while the LINE OUTs go to the SSV3 for me stage mix? Eliminating any need for a direct box anywhere? That could solve so much for me. Do I have that right?