Sonus Faber Cremona M's - add a subwoofer ??? room is about 13 feet across and 20 feet long and I have the speakers on the short wall. The room is treated with ASC tube traps......I have seen it once, seen it twice - you get the idea where Sonus Faber Cremona M's -floorstanders, are matched with REL sub's. Spoke with the nice people at SUMIKO-Sonus Faber and said the way the REL's are set up you can easily add a sub onto the floorstanders and greatly improve the musicality of the speakers. I have NEVER had subs, always thought that they would ever be integrated properly - but I guess the product has come along way. The REL's have internal crossovers that will match with your speakers and room very nicely.......well your thoughts. No need to get into subs but I have heard that the REL's do go well with Sonus Faber......I know about JL Audio, Vandersteen .....let me know your thoughts
One question you might want to ponder is whether the money invested in the sub could be better spent elsewhere in your system. Your room is on the small to medium size--very similar to mine (17 X 13) and you have pretty close to full range speakers. I've heard the Cremona's before (extensive audition at a dealer) and did not find them AT ALL shy in the bass department. It has been said that adding a sub improves performance through the midrange (and some even claim the high frequencies). How much improvement can you expect with the Cremona's, which are very detailed and transparent in this range already is an open question. I ran a REL Stadium II with my Harbeth C-7's at one point (MUCH larger room) and it was beneficial but at the end of the day a larger floorstander (Daedalus) proved more coherent than trying to blend the sub with the standmounts. In sum, with your room and speakers I wouldn't do a sub. Rather, I'd look at the rest of my system and see what could be improved in front of my Cremona's and go in that direction instead.
Of course the Sumiko/SF people would recommend REL. Sumiko distributes REL as well. Cremona Ms are extended and flat right down to 40 Hz and drop off pretty sharply after that. So for most music they sound just fine, but if you listen to orchestral or prog rock driven by a 5-string bass or synth or pipe organ, getting full measure of that last octave would be a good thing.

I don't know what your budget is, but did you know that Sonus Faber makes their own powered sub in the Cremona line? Not cheap, but should be well-suited for integrating with the Cremona Ms.

Also, if you want the sure bet for the sub that integrates, that can match power output, speed, and bass extension to your speakers, it's JL.

Other contenders would be the better offerings from Martin Logan and B&W (their PV1D is really impressive).
I was going to get the Cremona's but while looking for a pair got a deal on some GamuT L5s that I couldn't turn down. I use them with a pair of REL Stadium subs and they work very well; the Gamuts are similar in size and frequency response to the SFs. The RELs are the easiest subs to match to main speakers and disappear to the greatest degree of any I have tried ; I currently have 3 other pairs of subs, all of whom are good sounding to me but the RELs are my favorites.