In a spare system in a smallish bedroom (my UHD video system, although it sounds quite good on music using my Panasonic 9000 playing files from the NAS), I use a Sumiko S.5 sub I got on close-out (with my old B&W P6s I own since 1997), which is a copy of a Rel T5 (Sumiko used to distribute Rel). The sub works really well in the space (11x13 feet with a 10 foot ceiling). I did try to get a Sumiko S.9 (which is a copy of a Rel T9) but I just missed out on the last one at the time.
Eventually, I know I'll have to replace the old Rels at some point. Upstairs in my bonus room (basically a guest bedroom), I have a pair of GR Research N3s as mains and when my old Sunfire, Jr. sub started to go (I could have gotten it repaired for $200-300), I opted to upgrade the master bedroom (which is strictly HT with in ceiling speakers) to a
Bic Acoustech PL-200 II. I had two subs in the bedroom. The KLH 10 inch 100W one someone gave me went and I moved an 80W 8 inch Dayton Audio sub upstairs to fill in the bottom end of the GR Research speakers. It works well in that regard (not that the system sees tons of use). At some point when I upgrade something, I'll move stuff around.
