I just purchased a ext 50 to use as a frag system to compliment my ext 170 im also working on.
I will run 2 xr15 blues. I was going to do 3, as I have 5, from my previous build. 3 was way more than I needed based on par from my last system that was the same height.
Anyway you can always turn them down, lol.
I will be doing the same, turning them down, running 2 RB 50's, on the 170.
I am using T slot extrusions to mount them for anyone that does not like the rms mounts. You can use Tnuts and bolts to attatch it to the stand if you want.
Keep the pics commimg as I am curious as to what levels you will run the 30's at.