Going to go single or double(yeah I know that beneath 150 hrz that you "can't localize) bass but two is what many folks do.Storm would be first choice.Or larger.But remember that a 20 Hrz wave is 56.something large so even in big room your not going to get lowest notes that you might hear ((25hrz is more reasonable).Think older series maybe a single stadium is good.They are fast multi drivers.Too bad Vandersteen never made a bigger 2WQ.That said and though not with same finesse Logan has a fine sub (some have said better for HT and music and not as refined as REL) but whatever you get Maggie told me get multi driver that does not go beyond 6" driver.ML Depth is really a fine sub.Since it's not going to reproduce a 20 Hrz wave in actuality your room may need more SPL than depth (ergo ML and two not one)Both they an REL don't have digital e.q. that other like Velodyne have so a REL you can afford (again older models still built not newer series or one from some guys who left company and started MJ Acoustics are worth looking into.Then get a SVS EQ with mike to smooth it all out since these brands don't have the EQ.Your on right track though knowing about REL and that speed not "Boom In The Room" is what a music lover wants.Me I'd go two Storms or ML Depth's and SVS-1.Wish I could name older model above Storm but you'll see it if REL is the choice.
G'luck
Chazz
G'luck
Chazz