The SVS NSD subs definitely put out a lot of bass for the $$, helped by their size (they're nearly 2x the size of the Axiom subs). They don't have any of the EP500 DSP-based features though (very flat response, graceful handling of overdrive etc..), so comparing to EP350 (same cabinet & driver as EP500 AFAIK, but not DSP) probably makes more sense.
Sub reviews sometimes rate subs very differently depending on whether they are being used for music or movies (movies tend to want "lots of bass", music tends to favor "tight" bass that stops quickly). You can design a sub that gets relatively more output from the port/cabinet combo and get more bass for the $$, but if you look at the graphs showing how response trails off over time at different frequencies you see the cost -- a much slower "decay" around the port/cabinet tune frequency.
If your requirements are mostly "lots of bass" for movies then non-DSP ported subs like the NSD offer a lot of performance for the $$. Note that Axiom also updated the EP350 recently with a larger cabinet and amp, although reviews of the Axiom subs suggest that they still lean towards a more "balanced" design with a bit less port output (therefore a bit less overall low bass output) but tighter response for music and movies with dynamic bass.
Or something like that