EP600 versus SVS PB13-Ultra
So... with that ugliness out of the way, below is a post detailing a comparison a few of us did between the PB13 and EP600. As an obvious disclosure, I am an Axiom owner. I purchased the PB-13 Ultra after having read all of the great reviews while I was still in the trial period with my Axiom EP-600 as well to do a direct comparison.
Further disclosures, I'm not claiming to be a subwoofer review expert nor do I want to incite a vicious argument of which sub is better (personally I think they are both great). Hopefully you'll see from the comments posted in the following links that the review is fairly neutral and that there was no question as to the dominance of the SVS at the low end of the spectrum. To me and the small group of friends that did the comparison together (not all of whom were Axiom owners), however, this did not make it the hands down winner, for reasons explained in the linked posts.
The one comment I would offer on the above posted frequency response curves is that I did not measure anything near that level of discrepancy in my own room. I've seen the curves above before and know they are from a respected source, but they are so different than what was produced in my room that they seem a little misleading as to what the average user will experience with the Axiom EP-600.
With all that said, below are the links. Hopefully you find them informative.
Listening tests and frequency response curves
Final Comparison & Decision
PS: I now have an EP-800 on the way as well, which I can't wait to compare to the 600. The shortcoming I have found with the 600 is the tendency to produce port noise when high volumes are combined with ultra low frequencies. Since the new 800 is sealed, that problem appears to be solved.