1 or 2 subwoofer for small room & placement

I have read that 2 sub-bass systems will yield better results compared to one in 2-channel stereo setups. Is this applicable to a small room of say 11.5'x17'x9'? Or one sub will probably be more than adequate and yield better results compared to 2 subs which may be too much for a small room?

I know experimenting is the key, but if sticking with one sub, which is the most preferred placement that will produce the best results? At the extreme corner of the room close to wall boundaries, or in between the main speakers at the middle in almost free space? I have tried both and slightly prefer the sub in between the speakers in free space. I'm curious to know which configuration is mostly adopted.

Lastly, are the active crossovers in the REL Stadium/Stentor subs made of high quality parts for use in high-pass configuration(in rolling-off bass to the main speakers)? Concern is with the sub's crossover inserted in between the preamp and power amp that may sacrifice sound quality if it's of inferior quality.

Any advice from knowledgable folks would be most appreciated.
IMHO, it depends...two low quality subs might cost the same as a single high quality sub but may not sound better. Obviously two identical subs should play louder and more accurately than a single sub - but integrating them will be more difficult. Another factor is room acoustics...a single sub may already be enough in a small room.
Good question. I recently began experimenting with a sub in my music-only system. The sub between the speakers sounds much better to me than the corner placement. My sub is a Velodyne SPL-800II, love the small size, but I also have concerns about using the subs crossover for the same reasons you state. The bass produced in my small room seems adequate but the thought of a sub on the other side is intriguing.
I use two subs, however...I've found that two are not really needed unless you are going to cross them over at a fairly hi crossover point, ie...80HZ and above.

My subs and mains blend great at around 65hz in my room. They are behind the main speakers, and you really can't tell if one is turned off...I've tested it in the past.

Subs should be located near the mains for music, I've never liked corner placement, although I'm sure corner placement works fine in some rooms...and with some subs (not mine though).

Corner placement of one sub can smear the soundstage; the higher the crossover freq the more it can imbalance. For my room the most musical location is centered between the speakers and with almost same distance from front wall as the mains. This enhances soundstaging and still good two channel bass. For home theatre and movies corner placement will give most impact. For music best arrangement for corner placeemt is two subs.
2 subs will load room better than 1.Most of my subs end up between the loudspeakers.
Thanks for responses. Looks like I'm not alone in getting better results with the sub in between the speakers. Just purchased a Radio Shack meter and would be curious to know how things can be improved even further.
I use a pair of Vandersteen 2WQ subs with Thiel CS2.4 mains. I don't know about other subs, but the 2WQs are tunable (volume and Q) and they mate very well with the 2.4s.

The effect in my system was not subtle. Yes, the low frequency material was there with good authority, but the sound all the way up through the midrange was cleaned up and tightened up; better timbral accuracy. Also, which was a surprise to me, the overall imaging and soundstage were much improved. These are qualities that Thiels are known for on their own anyway.

With the 2 subs its a whole new class of speaker system. Like I said, my impressions are solely in reference to the 2WQs.

In regard to placement, I think just put them where they fit best and tune them to that position. Mine are a couple feet behind my mains.
In my large room, with 7 channels, I have the main L/R mounted on top of 15" subs. This, in effect gives me 4-way mains. Works great for HT and multichannel music.

But in my smaller room, that has a turntable setup I only use one sub, which is in between the two speakers.

In the first room that I ever used a sub I had the sub in the front corner. That room had bad standing waves, due to a steep vaulted ceiling, that topped out at 15' and the wall then dropped down to the 8' level.
So it really comes down to what the room needs.
Multiple low frequency sources spread around the room either symmetrically or asymmetrically (my preference) will result in smoother in-room bass. This is in part because there is a unique peak-and-dip pattern at the listening position for each subwoofer location, so the sum of multiple subwoofers will average out smoother than any single unequalized sub. This smoothing effect has been demonstrated by several researchers (Welti et al, Geddes).

I place two small subs in an arc formation, between and slightly in front of my monitors to very great effect.
Going by your room size you don't need 2 subs. The problem is you will be able to locate the sub if it handles frequencies above 40hz i.e bass is directional unless you are taking about low frequencies say 40hz and below. Its in this situation where 2 subs would be better. If you are setting the XO to less the 40hz you should not be able to locate it. Now you can just as easily just plunk the thing in the middle in between two main speakes to sort of solve this problem.
The REL you mentioned I believe is downward firing so this also helps in the spreading the sub-bass rather than allowing the listener to locate it.
Traditional placement of a REL sub is a corner, facing or firing diagonally into the room. So again you may be able to get away with just one.
Go with a single sub and see how much of the location problems bother you.