Assuming cosmetics isn't the highest priority (because I'll admit the crystal acoustic sub is a looker) I'd spend a bit more $ and recommend the HSU VTF-15H for $1000, or the Epik Empire for $800, or the SVSound of your choice in that $1000 range, over the Crystal Acoustic sub for $750. (all of these subs I've heard and measured in my room, and in my opinion/experience the Crystal Acoustic is the weakest of these options --- though it is the cheapest). It won't get you to reference in pairs, their THX cert requires four of them. (see this link - http://www.crystalaudiovideo.com/ProductCatalog/Speakers/Subwoofers/THX.aspx
The Crystal Acoustic sub doesn't scale very well either IMO. If you are spending $3K on these subs to get four at $750 a piece for THX Ultra spec, you are spending $3k poorly compared to alternatives like the JTR S2, the JTR Captivator, the Seaton Submersive HP, a pair of LMS-5400 or UXL 18s, or a FunkyWaves LMS-5400 setup. Each of these alternatives will get you well beyond THX reference level bass with just a single unit. (Albeit much greater expense ---and sans the official licensed THX badge). THX certification simply specifies reference level volume to 20hz (-6dB), which is a easy goal post that each of the aforementioned subs exceed for less or equal to the total $3k.
As I mentioned earlier. No two subs retail subs or ID subs for $1000 are going to get you to reference flat level bass in a typical home theater room. You can't even do it with DIY I don't believe. If there's a way - lets talk. KCNitro07 just spent $1k on DIY setup and he's not getting reference level bass in a small room. He used Dayton Audio Titanic MK III pair of drivers, in a couple DIY cabs he made, with some deckrestore as the cosmetic finish, and a Inuke DSP 3000 amp. Total price is right at 1K. Sounds good - but not reference, even in his small room. He can't take the volume much over -8dB without encountering clip lights on the INuke DSP amp. I've measured with him, so I know first hand.
Best pair of subs for $1000? Hard to say. I'd look at Epik or HSU or Outlaw -- there aren't many games in town. You could also do what KCNitro did for DIY - that worked out okay.
He's fairly flat in room to 25hz, and then drops by 5-7dB to 20hz using REW calculated DSP on the Inuke. Almost THX spec if he could actually hit 109dB at 20hz which isn't going to happen with just a pair. He'd need four and that's $2k, not $1k.
If it were my $1000 and that was my hard ceiling - I'd buy a HSU VTF-15H and call it a day (not reference level but a darn good sub for right at $1k. Add a second if you find you need it and when you can afford it.