I had the exact same problems with my SVT4Pro, it just didn't get loud enough no matter what I did. I used it with svt 410hlf & svt 610av but there wasn't enough headroom with either of those cabs. AND we didn't play loud. I don't know if all these amps have same kind of problems but mine didn't get even close to those 1200 watts what should be getting out of it.
Now I'm using MarkBass LMII and I have to set the volume to 9 o'clock max. I have also A/B:d 4Pro with SVT CL and CL was much, much louder than 4Pro (and yes of course better sounding). It (4Pro) was wired correctly and everything, I just didn't get rid off the limiting problem. No matter if master was all the way up and gain was used to control the volume. So I got rid of it...