Should I get a second sub or a higher quality single sub?

I have 600W mono-block amps and excellent speakers.  I recently purchased a JL Audio e112 sub.  The e112 blends seamlessly with the speakers giving a nice big tone and not too much bottom.  Actually, I never really "hear" the sub, it just seems to extend the speakers lower frequency range.  The only problem is that when playing the system at moderately loud levels the volume of the e112 must be turned up to the max.  I know the sub is operating properly because I returned it to the dealer to be checked out, so I am not sure exactly why that happens.  

So, I have been considering whether I should purchase a second e112 sub, or sell it and upgrade to a JL Audio Fathom 112 (the Fathom series of subs is better than the e series).  Does anyone have any advice whether I should get a second sub (an e112) or get a better, more expensive single sub (the Fathom 112)?  My listening room is 25' long X 12.5' wide X 7' high.

..the more subs the better and...having the best sub made isn't nearly as good as 2 less capable, less expensive subs
Contrary with me,
I found single Firehouse sub is much tastier and better than double Subway one.
Post removed 
Connected via outputs from the pre-amp.
Post removed 

As a general principle, the in-room smoothness in the bass region improves as the number of independent bass sources increases.  So two subs intelligently placed (far apart instead of close together, for instance) will have about half as much variation in in-room frequency response as one sub.  And four subs will have about half as much in-room variation as two subs. 

One of my four-sub customers recently had a technician come over to calibrate his powerful room correction system.  He started with all controls flat.  The ONLY correction the technician found to be needed was a simple level adjustment - the in-room response of the four subs was inherently smooth enough that no further EQ was needed!  The technician said that he'd never seen anything like it.  And this smoothness holds up well throughout the room, unlike EQing a single sub, which usually makes the response worse at other locations within the room. 

The relevant point being, two subs intelligently placed and properly adjusted will be much smoother in-room than a single larger sub.  And smooth bass = fast bass.   (Because low frequency systems in rooms are essentially minimum-phase, when we fix the frequency response we have also fixed the time domain response, according to physicist Earl Geddes, from whom I learned about the advantages of a distributed multi-sub system).



Duke is spot on - the more independent bass sources, the smoother the in-room response. As much as I would love to explore this option, until I have a dedicated listening room, it’s just not going to happen. My only option was a single sub, and I went through a few - Velodyne, Vandersteen, SVS, JL Audio, and Seaton. The best option - in my room, to my ears - is my Seaton Submersive (by a wide margin). I’ve never owned, or heard for that matter, a sub that is as powerful, detailed and musical as the submersive.
I prefer using a minimum of two. I have had as many as four 8" drivers fashioned in individual closed boxes. That said I don't have a set preference on hook up methods I find it can depend on system/s. Sometimes the HLs make things easier to set up and also sound better in the end. And versa-visa. I have had a bunch of subs and like a bunch. Also look into Rythmik. I'm am currently awaiting a venture into two "home made" Rythmik servos ( a sprained forefinger can be a bigger deal than it would otherwise seem). I convinced a friend to go their way and buy two of their sealed subs, glad I did. More importantly so is he. And a + 1 for Seaton.  
Thanks for that, Duke.  Informative!