I remember when I upgraded from my entry level SVS to the PB12-Plus I have now and at first I wasn't blown away either but did notice some gains. Later on, I was selling my previous sub and I had to demo it for a potential buyer and boy was I shocked at the difference going back, big difference. I think it just speaks to the high quality all SVS subs are and how difficult it is to make improvements. While they are there, you have to listen more to hear them.
Keep playing with the setup and do get some measuring tools/software. Not directed at you but I'm shocked at the number of people that don't want to invest the time to calibrate the sub correctly and continue to complain about how crappy their sub sounds.