Subwoofers

my first sub question is always, how do you plan to hook them up and do you plan to highpass? 
 

If you can get past the looks the JTR RS1 the best sub I personally have heard that are not diy monsters. Their lowest low pass setting is 60hz… so keep that in mind if you are not using external controls. I am highpassing mine with a Sublime Acoustics K235.  
 

I am not a fan of Rel. too expensive for round rolled off bass. Their top model are probably fine if you want the looks and have money to spend. My JLs had a lot of failures and I would not recommend the brand. They do sound good though. Rythmic and HSU both look like good buys. 

My room is approximately 20 ft x 35 ft.  I'm using two REL S/510 in a stereo pair.  Using speaker level connections for two channel.  I have the gain and crossover set really low. They blend well with my full range speakers. 

If I turn them up very high, it's too much.  I can't feature needing bigger subs for two channel, but I haven't heard them. 

They don't have enough oomph for home theatre. I have a larger SVS sub that kicks in when I'm watching movies 

The S/ 510 and 812 are excellent subs

@greywolf In a room that size you are going to need two 15" or four 12" drivers at work. I design and build my own subwoofers and I only recommend the brands KEF, Martin Logan and Magico. I think what would best suite you would be two Martin Logan Balanced Force 212s. Balanced force subwoofers have matching drivers at opposite ends of the enclosure working in phase so both cones extend and contract at the same time cancelling out all Newtonian forces.The subwoofers produce less distortion. At this time REL, SVS and JL Audio make nothing like them. If you are going to do subwoofers a two way crossover is IMHO mandatory to get the best performance out of the system

Well Rhythmik has a foam surround, the Martin Logan costs to much. Still undecided.