Zak, I thought I had replied already. Sorry.
I have an older SVS PB12-NSD. It is a very good sub and has been completely trouble-free.
I had the good fortune to hear the PB13+ and EP600 side-by-side at Jason's (myrison) home some years ago. Both subs were fantastic, but Jason kept the Axiom. I would be very curious about the sealed SVS subs; their ported models do seem to have a distinctive sound that is perhaps a bit less articulate than some other subs I've heard. Man, they play LOW, though.
The ported SVS are really big boxes.
How about the SS-18.1
(18" sealed) from Chase Home Thater? They are using the Dayton outboard amp, which seems to be very reliable, and makes problem-solving much more straightforward.
That might be my next sub.