Two subs, yes or no?

Overwhelming yes! Two subs fill up your space faster, and as bass waves move slower, this will make the bass seem lighter and faster, even if you went with cheaper subs. The trick will be dialing them in so that they mesh well with your mains and then making them work in your room so they aren't just "one note" bass boxes.

Hey thanks guys, this is very very helpful.

I was at my favorite dealer Ovation Audio this weekend and Mike and Rickey spoke of running 2 x mono to integrate with my xls360's..
Two things as I was not clear. When I make reference to using both rcs and balaced output at the same time, in fact it is rca and balanced out of the variable output.

The other thing is Swampwalker suggests the position as asymmetrically (one inside left main, one outside right main, for example), which is weird because this very suggestion solves my left corner where there is a door to my balcony! Problem solved.

You will have to experiment for sure, but the idea of asymmetrical placment is to (theoretically) help with room modes. If both the balanced and RCA variable outs are active, that will help, but you may have to compensate for differential gain via se and balanced circuitry IF it is truly balanced and not just an XLR connector wired to se circuit. If not, then the differential gain issue disappears, and could be handled of course when adjusting sub gain to match mains.

Just my 2 cents.. If you can track down a pair of Rocket UFW-10 subs you'd be very happy.. Best small sub I've ever heard, I picked one up a couple years ago (only needed one for the size room we have) and I couldn't be happier. Incredible performance for a smaller sub, if your mostly into music it's just perfect, it's quick, tight and can go deep and as far with HT I have absolutely no complaints, does a great job. I still have some of the reviews bookmarked I'll post here, only problem is I see they no longer build this sub, not sure why for it was one of they're biggest sellers.. I have the Rosewood finish model, has a glossy Piano black top and the included brass spikes with discs under them(hardwood floor protection) only reason I mention the finish & feet is because I have this sub in our main living room and it blends in nicely, not like most subs that look like a black box in the corner.. Actually my wife has a tall artificial Fiscus Tree on top of it, believe me if looked bad or out of place she wouldn't be shy letting me know :)..

http://www.goodsound.com/equipment/rocket_ufw_10.htm
http://www.audioholics.com/reviews/speakers/subwoofers/av123-rocket-ufw-10
http://www.hometheaterhifi.com/volume_11_2/onix-ufw-10-subwoofer-5-2004.html