Craig -- I think it is probably a fair historical statement that Axiom subs are less often paired with non-Axiom speakers. In other words an Axiom sub owner has a high likelihood of also being an Axiom speaker owner, whereas there are other manufacturers known primarily for subs (for example HSU, SVS, PSA), which are more likely to be paired up with speakers of a completely different make.
I've got 2 EP-500's on the way (layaway), for reference. I also considered two PB-2000's, and have generally only heard good things about SVS subs. I don't need them to be bad in order to justify my EP-500's.
One thing that I find under-discussed with regards to Axiom subs is the analog power supply. Strange that devotees of various cults of audio purity (only 100% air-cored inductors or thou shalt be shunned and disfellowshipped, etc.) don't seem to be impressed by the pronounced benefits of this upgrade for dynamics, longevity, and elimination of switching frequency artifacts. It is important to note also that this is not a cheap upgrade, and that it is likely a substantial cost driver in the price / output specs race. In fact, the use of this part (the value of which the Axiom engineers seem to be adamant about) really undercuts the absurd claims of cheap parts / penny pinching that are sometimes circulated elsewhere. I don't know of any other mainstream sub manufacturer that does this at similar price points.
Second comment on the EP-500 is that it has now gone sealed. Although output SPL's at moderate amp power generally require ports, the argument for the new sealed design is lower distortion, more elegant filtering (driver loaded at all f's by back pressure), and (obviously) elimination of port noise. Sealed designs also generally seem to be considered faster and more musical. Andrew and Ian have also touted their limiter design, claiming that it very effectively suppresses the output of strange grunts/growls as the circuit is engaged.
Last thing -- it is sometime suggested that the steep frequency slopes on their crossovers promote time-domain ringing at the cutoff. I'm a bit skeptical of this. It would seem that more severe problems with time localization would arise from an absence of higher frequency modes as opposed lower --- and it is the high pass (at low frequencies) that is steep, not so much the low-pass (at high frequencies). Moreover, most users will probably use their own external bass management on the low-pass integration with mains. It would be interesting to see some analysis / listening tests / measurements of these issues. The comments I've made here are "top of the head", and may well be somewhat naive.
Cheers -DSQ
PS -- I'm sure most interested parties have probably seen Andrew's excellent subwoofer article at S&V
here, but I'll link.
