So I had a Brio R with a sub attached to it. You are thinking the right way looking at nice, sealed subs. The Rel is the best of the bunch but that neutrick cable you will need to attach it is like $450. Stupidly expensive.

The SVS SB-2000 lacks speaker level inputs making it harder to install. I may not be remembering this right but I thought that when you turned on and used the tape loop you lost other functionality.

The Velodyne Imapact 12 is a very good sub for home theater, has the right inputs but is ported and not the most musical. You will sacrifice bass quality with vinyl. Read the WhatHiFi review.

The BK also uses that neutrik connector so it will be expensive to attach to your system just like the Rel.

I personally like the Sunfire HRS line. You can get a brand new Sunfire HRS 8 from Amazon for $699. Crutchfield has it for $849. I can say that this is an extremely musical sub that can be attached to the Brio with speaker wire easily. It is 1000w of power and goes to 22hz in a lovely piano black finish. Accessories4less has a great deal on an open box of the Sunfire HRS 12 which is also 1000w at $649. I had that exact sub in a home theater and it was described as sounding like a truck was driving through the house yet can be ultra refined and deliver the finest detail. They are great subs for the money and like I said, can be attached with speaker wire as opposed to having to get those viciously expensive neutrik cables.

Rythmik

http://www.rythmikaudio.com/