I'll start off upfront saying I have only heard one of the subs you mentioned in your original post. I own a SVS PB-13 Ultra subwoofer. It is a finely built sub with tremendous output. It also happens to sound good

. Look at the second picture, that is six Ultras owned by Mike C.
http://forums.audioholics.com/forums/showpost.php?p=192644&postcount=1
If you are trying to stay within 1 grand, go for the Epik castle. It will have a plain finish, but has the performance of an Ultra, as rated by Craig. If so, it will easily fill your room with bass, as my Ultra has little problem in my large master bedroom.
Another member bought an Epik Tower and he was blown away by the output. It just a few dollars more than the Castle ($100.00) but slightly larger. Bear in mind, all subs in this post but one are quite large. A much smaller but sealed 15" is the Hsu Research ULS-15. It's a few hundred dollars more and new on the market. I've owned a Hsu sub, a discontinued model, the VTF-3 HO Turbo, a 500 watt 12" ported sub that was top quality all around, excellent for music, and with great output for movies. The ULS-15 has caused quite a stir among board members because Dr Hsu builds some serious subwoofers, and this one goes down to 10 Hz. and is quite small size wise. Dr. Hsu is serious about sound quality also.
http://hsuresearch.com/products/uls15.html
Sorry I know nothing about the Rythmik DS 1500 kit. But I hope I have given you some useful information.
Ric
*Edit*
Another Hsu Research sub to consider is the VTF-3 MK3 @ $699.00 for basic bllack, $799.00 for veneers.
http://hsuresearch.com/products/vtf-3-mk3.html