At the moment I own two ep600s, two ep500s and a PC-Ultra which I am in the process of implementing into a quad sub configuration with the help of a Velodyne SMS-1. They are all great subs with their own strengths and weaknesses. Two ep500s are on par with the Ultra/2. The 500s have a more linear frequency response while the Ultra has uneven response with a hump in the mid-bass declining by around 10bd over the audible range. Here is a graph in 4pi using a 95db sweep done last October comparing FR of an Ultra tuned at 20hz and a 500 done last October with the help of Ian and Tom.
The 500s extend only to 19hz while the Ultra's tuned at 16hz go down to 11hz. Overall the Ultra has lower THD compared to the Ultra but their smaller amps and large excursion drivers produce more IM distortion. The 500s sound somewhat cleaner and more detailed but lack the Ultra's extension. At the listening test which I attended overall 5 of the 6 listeners preferred the 500s for movies and music with one listener (Ajax) liking them about the same. I found the 500 and Ultra in 16hz tune only very close in sound quality but preferred the Ultra for movies because of its extension and the 500 for music because of its greater detail. The Ultra in 20hz or 25 hz tune just doesn't sound as good as the 500. So the decision should focus on whether you want more extension in the infrasonic band with the Ultra or a slightly more detailed linear sound in the audible range with the 500.
With two 600s the edge goes easily to the Axioms owing to their greater headroom and the 600s more linear extension from 16 hz to 100hz. Here is a graph of the 600 and Ultra.
One combination to consider is the 500/600 to the Ultra/2. This combo is similar to the two 600s except there is 3db less response at 16-19hz but saves you $600. Those two subs blend nicely without the usual phase/delay isues found when using dissimilar subs.
Hope this helps.