Long time lurker here but I thought I would put my two cents in and echo what the OP said. We recently added a REL S-SHO 5 to our two channel system and it just makes everything sound better. In addition to improving the low end, what I had perceived as brightness in the system is now gone. The imaging seems more stable as one moves around the room and interestingly, it sounds better at low volume as well. I think it's more than a placebo effect. I don't understand the physics of why it just sounds so much better overall but couldn't be more pleased. It sounds like a whole new system. For those interested, we have Symphonic Line Legato speakers powered by Odyssey amp and preamp. Large room at 17x35 feet with vaulted ceiling. I'm sure similar results with other subs but REL was recommended by our local dealer who brought one out for an audition.
Jim
Jim

