I love my LS50/Nuprime combo - soundstage detail etc with my HD-AVA amp without going towards huge floor standers. However I would like to explore some more low end (LS50 has a response of 79Hz -3dB). What sub would work well with my setup. My living room is 20x30 (speakers are 10ft away in the center and 1ft from the 20' wall and my couch is 10ft away from each speaker (triangle).
Couple of things - First - you're wonderfully spoiled with a full range driver. Mating it to a sub will be a nightmare if you don't do it right. Trust me - I lose sleep with my designs.
With your 10' triangle - forget the room - focus on that area only. You don't need a large sub. You also don't technically need a sub - it's more important that the driver is fast and integrates with the LS50.
I don't personally like a lot of options out there. The players are boring if you ask me. I'm going to suggest REL. Some great stuff - although they are a bit snoody.
The REL T/9i is offers the best performance/price you can get. It's a great match for my Ruby. It's also available in white. While it's not cheap - it's the right sub for you. Your LS50 and other gear weren't cheap - check it out I think you will be impressed. rel (dot) net