Yeah, the reason I'm not recommending the HSU VTF-15H (or a pair of them) is because the VTF-15H is, IMO, the worst sub that HSU makes. Now, don't get me wrong, it is NOT a bad sub. I'm just saying that out of all of HSU's subs (OK, maybe excluding the 3 smallest STF-1, STF-2 and VTF-1) the VTF-15H is the last one I would take, personally.
The problem with the VTF-15H is that it is not linear at all. Oh sure, when it isn't playing too loud, it'll play linearly just fine. But when you start to up the volume, you get this large hump around 40-50Hz where it cranks out the volume, but above and below that, the frequency response drops way down. And for the really deep sub-30Hz stuff, there's just a ton of distortion and not nearly the output capability of an SVSound Ultra or even a Plus.
You've gotta realize that the VTF-15H uses the same 350Watt BASH amp as the VTF-3 MK4 and even the old VTF-3 MK2. The SVSound 1000Watt Sledge amp found in the Ultra series is on a COMPLETELY different level of quality and capability, as is the Ultra 13.5" woofer, which is just miles and miles ahead of the 15" paper cone in the VTF-15H.
The 15H just runs out of steam. You'd need 8 of them to equal the 20Hz output of the PC13-Ultra with the same low distortion. Is that cheaper than a single Ultra cylinder?
There's a reason why the prices are the way they are. SVS isn't gouging on the prices here. For one, they're including shipping, which on subs the size and weight as theirs, is a couple hundred bucks easy. But more than that, it's the parts quality.
Most people aren't ever going to come even close to needing to actually crank the VTF-15H. That's why a lot of people are perfectly happy with it and think that it can play amazingly low and loud. But the OP made a PC12-NSD DSP run out of steam. The VTF-15H wouldn't fair any better - it's about equal to the PC12-NSD DSP except that it doesn't have the NSD's DSP amp to control distortion and linearity.
Like I said, the VTF-15H is not at all a bad sub. Neither is the PC12-NSD DSP! They are both good subs. But they are not the equal of an Ultra or Plus SVS sub.
As for Axiom's subs, the EP500 is the only one worth a look. In terms of quality of the sound, it's around SVS Plus or Rythmik levels. It just lacks the output. But it has a physically smaller size, so that's where it might make sense. The EP600 sounds like *** and the EP800 isn't much better, so it really is all about the EP500 with Axiom. It costs about the same as a SVS Plus or Rythmik and quality-wise, does about as well, so it isn't way out of line. You just trade max output for physical size, so I can still see value in it. That is, until its amp inevitably starts acting up on you