Sub for 2 channel music only setup

Agree with Stanwal and Kal on the REL. The REL is one of the only sub systems to use hi-level inputs from your main amp with no crossovers in the signal path of your main speakers. When set correctly, you only notice the presence of the sub when you turn it off - the extremely low bass disappears and the soundstage collapses. For best performance, it is critical to use one of Synergistic Research's Tesla Rel Spec cables for the hi-level connection.

Another vote for Rel. Personally I stay away from the 'R' series.
I recently upgraded the stock cable to a Nordost Baseline and that made a huge difference. I had no idea what I was missing

I have tried MANY subs, and finally tried a Rel (B1), and it's absolutely the best all around sub that I have had. It does 2 channel excellently, and with it's proprietary input options (both high and low level inputs at the same time) handles Home Theater like a champ.

I think the Vandersteen 2Wq was the best 2 channel sub as far as integration, but the Rel goes much deeper, and is just as fast.

The JL Audio F113 and Velodyne DD15 (with the SVS PC Ultra 13 right behind) were the best Home Theater subs I used, but they were limited on hook up options compared to the Rel's for 2 channel.

Rel is my recommendation, the higher up the food chain you can go, the better.

I had 2 Rel Stadium III for my HT slash 2 channel system. I felt the Stadium III was not fast enough and went too low for my 2 channel. I do not like subsonics when I listen to music. I sold 1 of my Stadium III after tring an Aerial SW-12. The SW-12 is much faster. It has an EQ, a left and right balanced input amoung other inputs and adjustments.
I connect the right balanced input from my 2 channel system and turn on the EQ so I can set it up just the way I want it. I want the sub to be there but not over powering. Then when I use it for HT, I turn off the EQ via remote (a macro)so I get full range from my 2 channel input and then a LFE signal from my HT processor to the left balance input for all movie effects. I took a while to get it where I liked it but it was worth it.
This setup has made me so happy I am going to get another SW-12 and have 2 subs in the front. BTW the Rel sub I still have is used in the rear.
For me I feel the faster the better when it comes to a sub for 2 channal, not how low it will go. How low do you really need a sub to go for music, maybe 25 HZ.

Hevac 1 - I'm not challenging your opinion, only curious: Did you use SR REL Spec hi-level cables and good power cords on the Stadium III's? Makes a huge difference.