Any of the subs mentioned above would be great performers in 98% of situations. I've auditioned both the EP600 and the SVS PB13-Ultra. The SVS goes a little deeper and a little louder than the 600, but lacks the DSP that the 600 has so there is a slight tradeoff between both speakers. I've also heard the EP500 and found it to be a highly capable performer, but with just slightly less ultra-low bass capability.
After doing extensive A/B testing with the EP600 and SVS, I started to appreciate more just what differences you hear between two quality subs whose specs are just slightly different (assuming they are both made by manufacturer's whose specs you can trust). The difference of 2-3 Hz extension at roughly the same volumes (i.e. the difference between playing flat down to 23 Hz or to 20 Hz) is extremely subtle, but for those who really live for the last few decibels of really low bass, it is a difference that can be worth a lot of extra money.
So... since my point probably still isn't clear after that long ramble, if you're looking for the best bang for your buck, the EP350 is probably the place to start (or any other competing subs in that price range, and there are a lot of good performers out there). Beyond that, the additional extension and volume you'll get from the more expensive subs (EP500,600,800) start to have slightly diminishing returns in terms of cost versus clearly noticeable differences.HOWEVER
if you're not looking for the best bang for the buck (i.e. you're looking for the ultimate performance and are willing to pay considerably more for that last ounce of rumbling bass), or you have an extremely large open room to fill with bass, then going up the Axiom product line will make you very happy. While I used to think I fell into the "most bang for the buck" camp, I'm finding that I am in an endless pursuit for the best thing out there (at least in the ~$2k range), which led to my pre-order of the EP800. The measurements on that beast are absolutely unbelievable and I cannot wait to see how it compares to my in-depth experience with the PB13-Ultra and the EP600 I have already. Playing down flat to 13 Hz with a sealed sub is something I'm really looking forward to.
Let us know if you have more questions as you get further along in the purchase/review process. Lots of folks around here have tried various other subs and should be able to give good first-hand information.