One Sub or Two?

For a 2 channel music system I recommend 2 subs. 2.2 sounds better than 2.1 in my experience. A lot of good manufacturers depending on what size you want and your budget. I have SVS SB1000 Pros which fit my space and budget. Love their app for control and their customer service for support.

Subs are available at widely varying price points and the lower you go in bandwidth or in desired SPL across that bandwidth the more you will spend. If your sub is supporting music reproduction only, there is very little point in having extension below 16 hz as that is the lowest frequency any instrument can produce as a fundamental to my knowledge. If you are talking about sound effects in movies, I am quite sure many movie soundtracks exploit frequencies below that though I have no intimate knowledge of which movies and how far below 16 hz movie sound tracks might go. You might take a look at the new SVS 17" sub while certainly not cheap, the one review I read on them was quite impressive claiming vanishingly low distortion at even sub-sonic frequencies. They also have a try it in your home for 45 days I believe. Further there is a control app for their subs that allows tweaking its performance from your listening position that is quite sophisticated. Between the app and their home trial, I'd give them a vote for being a low risk purchase. I have a pair of their PC2000 subs in my office/music room and love them. Not cheap, but quality usually isnt.

Like most of the others, I say get 2. As for which ones to get, my first choice would definitely be Vandersteen. If they are out of your price range, REL would probably be my 2nd choice.

30X40X12 is a very large room - 14,400 ft3, Two big subwoofers are a necessity to fill a space like that. My only complaints with REL are lack of continuous phase adjustment and their ignoring basic best practices for biamping. To be done properly, there needs to be a line-level crossover that rolls the mains off and rolls the subwoofer in, No speaker-level driven subwoofer can do this. Most HT Amps and preamps accomplish this via a Low Frequency Effects (LFE) output and Bass management - adjustable crossover frequency, level, and phase, often combined with Audyssey or Dirac equalization.. Some sub manufacturers put these electronics in the sub, but this requires 2 sets of interconnects - to the subs and then back to the main power amps. Certainly less than ideal, given that the subs may have to be located some distance from the preamps and power amps. This is problematic electronically, and the interconnect costs may be very significant.

As for me, I am awaiting for a high-end preamp with proper LFE and *shocking* a variable loudness control. Yamaha has demonstrated the utility of that for decades. To their credit, Parasound comes about as close as I have been able to find.