There are a number of factors to consider when it comes to bass.
To start, you want nice, linear extension all the way down to 20Hz if you're really after the full audio experience. You also want to achieve that linear extension with low distortion.
If you only have one seat that you really care about, you can carefully place a single subwoofer and then use active EQ to take care of any particularly nasty peaks at that one, primary location. If you have several seats that you care about though, then you really need at least 2 subwoofers. In order to achieve something close to linear response at several seats, you have to use multiple subs.
After thinking about linear response at all of the seats you care about with extension down to 20Hz and low distortion, you also have to think about output. There are quite a few subs that can deliver linear output with good extension and low distortion, but they can only do so at lower listening levels in the 80-90dB range. Crank up the volume and they stop being linear or they start to skyrocket in distortion or both!
Naturally, you need more output in a larger room. But you also need a lot of output regardless if you listen at reference levels and want to hit those 105-115dB LFE peaks!
So, to me, the "1 large sub vs. 2 smaller subs" question is really all about asking whether you need very loud output, but linear response at only one seat; or quieter output, but linear response at several seats. Of course, two large subwoofers can deliver very loud output with linear response at several seats
The SVSound PB12-NSD is the most impressive relatively small, relatively inexpensive subwoofer that I know of. The Epik Sentinel is also extremely impressive for its relatively low price and relatively small enclosure.
Both the PB12-NSD and the Sentinel can output upwards of 105dB (from 6 feet away) whilst maintaining flat, linear response all the way down to about 22Hz and keeping the distortion to around 10% THD. That is
amazingly good performance and there are many, many subwoofers that are far larger and cost WAY more that cannot match that kind of performance.
The even larger and more expensive SVSound and Epik subwoofers can play even louder and offer even lower distortion - and really, that is all that you are paying for and all that you could hope for!
At 600 sq. feet, you've got a large room. But using two subwoofers is more about getting linear response at several seats than it is about getting louder output. Of course, that is assuming that you position the two subwoofers at different locations in the room in order to gain the benefits of flatter frequency response at several locations. If you co-locate (ie. stack or put side-by-side) two subwoofers, then you are basically just making a "bigger" subwoofer and upping the output, but not doing anything to flatten the frequency response throughout the room.
