Hey Dan, if you're close to Southern California, you can hear Rythmiks at speaker maker Ascend Acoustics (in San Clemente), who have a distribution deal for the subs, which they partner with their speakers. Rythmik is located in Austin Texas, and you can pay them a visit. And Sterling Sound (the NYC audiophile-quality mastering house) have three pair of F15's in their systems. And, as I mentioned, Jim Salk buys the DIY kits from Rythmik and installs them in his larger speakers. He's in Pontiac Michigan.
If you can't hear them before buying, Rythmik has a money-back trial offer, you paying the shipping should you decide to not keep them. I've never heard of anyone returning them. If you're going to have a single sub, the new Rythmik 18" (sealed or ported) would be killer. Massive output, with the same sound quality as the 12" and 15" models.