The compressor on the X7 is unique because it only effects the clean low-passed side of the signal. Since the drive of the high-passed side inherently compresses that part of the signal, the clean compression on the other part of the signal helps the two signals blend together more organically.
Your DBX will compress the entire signal either before or after the Darkglass (depending on your chain), which will have a different effect than the X7 and depending on where you put it (before the Darkglass it could potentially smooth out the signal going into the overdrive and smooth out the drive character- maybe too much- and after the Darkglass it could smooth out the entire signal- again possibly too much depending on settings). I personally find the X7 when on to provide plenty of compression for my taste, but I also generally play through tube or hybrid amps with inherent compression.
The EQ on the X7 is very helpful for sculpting the character of each side of the split signal- I find myself bumping highs and high-mids generally to add more bite to the driven high-pass signal
