I listened to the PB12-NSD, before purchasing my PB13 Ultra and if the ISD is anywhere near the same output as the NSD I would say the EP500 is only slightly better than what you currently own, YMMV. But with the Axiom subs you get a smoother, richer, more life like sound, not the dry thud like most subs.
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Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70