One Sub or Two?

1) most definitely two. evens out the response, increases dynamics and avoids localization.

2) I had two REL T9i and they sounded very good but missed the really low end notes and authority I did not know existed until I tried the SVS SB 3000. The SVS SB 3000 is a wonderfully musical sounding subwoofer with that really low end power and authority that makes it a real subwoofer and an exceptional value,

30x40 feet? Start saving for S class RELs.

I have one Rythmic 15 in sealed sub. No space for 2. The speakers are high passes at 50 Hz and the sub is low passed at 50 Hz. 
 

The sub integration is seamless and you really can not local the sub listening.

I use 2 REL subs, had 1 for year then added the other and significant improvement... a single sub can sound quite good if you have a single listening position you can place the sub for...

For a room of that size, consider two (or more) Rythmik F18 or G25HP subs depending on what budget and room (and wife) constraints allow.