sub in small room with bookshelf advice needed

My bookshelf speakers fill my small room well, however I am interested in adding a sub but uncertain if it will just cause boom. In other words, I am using bookshelf speakers due to the fact that larger speakers sound bloated...can I get a sub to cover the 20 to 40Hz gap and adjust in the gain to the point that it blends but does not boom?

Speakers: Usher BE-718
Room: 9x15x7 (WxLxH)...I know the Ushers are a little much as is for this room...

Also, what sub is quick enough to blend with the Ushers?

Get one of the smaller S series RELs and drive it off the amp terminals. They offer almost unlimited adjustments.They will not interfear witk the Ushers but will add weight to the bottom end.
No the Rel.
The sub will allow a little more flexibility with speaker placement by even just eliminating the very low frequencies. You won't be able to shove the sub in a corner though.
excellent call on the Rel
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I'd say yes as well. BR makes good points. Being able to xover the sub and place it accurately and control the phase will be of immense help. I noticed a good many subs won't cut off under 50-60hz, until you get well into the sub makers upper tiers of subs though.

If you can get an eight or ten inch sub with that low a xover I'd say go for it. Depending on the $$. I've got a 11 x 14 x 8+ room with a old 10 inch vel and I use the xover on my receiver set at 60hz with some little Cantons and it's pretty seamless and reasonably tight sounding too.

Have fun.
I have a REL R305 in a 17 x 15 x 9 room and it is more than enough and sounds great. The internal crossover is very good however I'm driving it via a Velodyne SMS 1. It replaced a Velodyne HGS 10 which was very good but the REL just sounds more natural, on paper it's specs are not as good as the Velodyne's but I prefer it for music.

I have known of REL since their begining, but the first time I had really listened to one was when I walked into a dealer who was doing a demo with a T2. It sounded great and I was looking for a sub so I asked him to pull out a R205 and then a R305. The 205 is great for small rooms and it probably best for someone looking for a high performance 2.1 system that is compact.

The 305 is great for a small to medium size room and probably isn't the best choice for a HT only system. I'm not looking for 10 Hz so it was a perfect compliment to a 2 channel system that's 80% music.
Yes you can but it may come at the cost of image smearing/timing issues. I know a lot of people will claim there is no loss of image at 20z-40z... to each there own. I found I often needed to cross the speakers around 60z (or higher) to get them to blend in. My best guess is by the time you get to 40hz you speakers are no longer flat. Not sure how they are in your room.
Frequency response

I once tried to do this a long time ago but stopped using the sub because of very small timing issues. I personally would skip the sub and look for speakers with a little more bass... Your room is a good size. mid sided towers should work well. You might just be sensitive to boomy bass?