will adding a subwoofer improve overall audio

I loved my REL Storm Mk III when I had it.

Yes, but I suspect without ever having done it, that you can 'oversub' a room.
How small is a 'small listening room'? CUBIC FEET counts.
Just for reference, check out HSU Research site which has a 'recommended sub' size chart based on room size. That'll give you the idea.

Even with my large system a sub does improve the overall soundstage--but it has to be integrated very well. Which means no inexpensive subs and crossovers. I'm using the Talon thunderbird and Rives sub-PARC to drive it and crossover. With a very high end system this works very well. I would think the JL would be a good mate for your system. I have heard some RELs work very well for 2 channel as well. I have not done enough 2 channel with B&W to be able to comment.

"....would a sub help?"

Maybe.............

Depends on the size of your room
Depends on the dimensions of your room......
Depends on the set up you can facilitate......
Depends on what sounds you are trying to add or correct....

A sub could add some bass
A sub could help you with bass problems if properly integrated
A sub could give your main speakers a better sense of 'body'

A sub will not add to your systems clarity - in fact it could cloud it up if not properly integrated......
A sub will not enhance the sense of soundstage - generally speaking that is more a function of mid/high frequencies and set up......

Rives is right - sub integration is neither easy nor cheap if you want really quality results.

FWIW, except to compensate for room problems with full range speakers where you can use sub positioning to help fix it (flatten out nodes/reduce boomyness) I'd always use full range speakers (even small ones) and run them full range and add a sub, rolled off below 60hz, to fill out the bottom end. This can be done, sub wise cheaper and easier because you don't have to deal with the effects of a cheap cross over with the main speakers.

All IMHO, but something for you to consider.

Unless your main speakers are capable of faithful ~20hz reproduction, yes a sub will help. In my experience imaging, and overall accuracy improves no matter the size of the room. A good sub can be tuned to fit, though selecting the sub with the room size in mind is a good idea. IMO it's more important not to have too small a sub, than to have one too large for the room. One that's too large can be attenuated using the volume knob.

In my experience a sub made a tremendous difference. I have large floor standing speakers but my setup is in a corner and the ceilings are lower than most. This is far from ideal but I'm probably not the only one with a less than perfect listening environment. So, I ended up putting subs, passive boxes, in the corner sitting back farther than the towers. They fire in opposite directions and perpendicular to the mains. Having this setup for a couple years I can tell you it is the solution for me. I feel that I have successfully changed the negative characteristics of the room and it really sounds great as a result. The overall sound is much fuller and balanced.

Only if it is a high quality sub, correctly installed and tuned, and matched with the correct speakers. No if one or more of the above are not met. A su is not magic, but has the potential to be. Jallen

Heck, I was all set to write something and then I saw Bob's post. So I'll just second what he said.

I agree with Bob. Get a decent sub and set your mains to cross at about 80hz (I know. It seems like you're not getting your money's worth out of your speakers. But it's the right thing to do.)

You'll get much more clarity and soundstage info.

I, like Bob, don't obsess about linearity measuements. I have (thus far) integrated a SVS PC Ultra 13 with my Martin Logan Aerius I's by ear with wonderful results.

My most recent tweak was to put the sub on sorbothane pucks to take the floor and walls out of the equation. I'm hearing the sub and not the room. If you can pinpoint your sub, it's too loud. And a good sub can't "create" bass info that's not on the recording in the first place.

Take a look at the SVS stuff for "bang for the buck".

In my experience the sub lets me hear the ambient sound of the room the recording was made in, if that information is available on the recording. Several tracks on Joni Mitchell's "Blue" are examples where you can hear the air in the room much better with the sub in the mix.
I also tweaked my sub by placing it on a 3 inch thick maple block spiked to the floor with sorbothane pads between the sub and block. This cleans things up considerably.

I have a relatively modest system in a small room (Sonic Frontiers pre, Classe amp, Rotel/MSB front end, North Creek Music speakers with Kimber and Cardas wiring. I added an M&K sub right before they went out of business. For me the addition added a new level of enjoyment to listening. I spent a lot of time fiddling with the set up, but the added reinforcement in the bottom octaves was a great addition. To me it really balanced out the sound and enhanced the whole experience. After this, I would say there is no substitution for a good, musical, bottom end.

I agree with most that has been said...the improvement in added weight and realism is impressive. I use a NHT X2 crossover set at 50 hz....simple to set up. My sattelites deliver quickness,dynamics and punch and the sub delivers the power & majesty...works great.

I am completely of the opposite opinion about using the crossover in the sub . Don't do it. Run your mains full and bring your sub up under them. Bob and I have argued this out before. The poster on the last occasion tried both and preferred my way. I use REL and drive the subs off the amp outputs, which is what they recommend. If you have this option see which you like. We could have another theoretical discussion of the merits of each approach but it would be meaningless. If you get a sub with the option try it yourself and if you don't the question is moot. In any case a good sub makes a very worthwhile improvement in sound.

I agree run your main speakers full range and your sub at 50 or 80hz and below....I triend using the mains thru the crossover...degraded the sound...bad idea.

I second Stanwal and Wavetrader, in particular in the context of your system. Bob's point is quite valid with satelites speakers or speakers with limited dynamics, but that would not apply to your particular situation of "full range" tower speakers. Running the signal through extra cables and crossovers will always affect it, usually by some form of loss, such as detail, image precision etc.
Given that you have a small listening room I am not sure if a sub would give you the improvement that you are looking for. You would need a real high quality sub and a precise setup to get this right. Have you considered a speaker upgrade instead? Good luck!

OK--you have lots of good thoughts on this thread about subs-It comes down to your system and personal preference whether you use a xover or run your mains with full signal and and let your sub or subs xover at whatever low pass you find works best--my stats are almost flat to 40hz(measured professionally)by xovering them at 50hz with a good active xover I get much increased dynamics etc--I use 2 jlaudio fathom 113s---they do not come with built in xovers --I have tried it the other way with the mains running full and the subs blending and prefer using the xover --see reviews of jl subs in absolute sound and st phile where they used xovers--there are no rights or wrongs just personal preferences and system issues that you have to find out for yourself--good luck Rich

I've only stumbled late unto this thread and as far as my personal experience with all sorts of sub systems goes, I find Newbee's advice excellent. For all it is worth I have nothing to add or subtract from it, except if properly set up, it may give you some more sound stage clues - not affect the sound stage per se - often more felt than heard, if a really good sub is integrated well with the rest of your system.

The Wilson benesch Torus will do all the above. It wouldn't improve the clarity of your speakers, but will the lower bass. Not a lot of buzz on it, though JV at TAS did an in depth review, pretty expensive at 10K. Since I am a dealer for WB I am sure my phone will be ringing with orders.

Boy in a small room wouldn't Gilman be better served to go with a better CD player?

I have Electra 936's and 2 JL Audio F113's and it is a HUGE improvement. I would have never bought these subs from reading posts or reviews. What I was reading however made me find some to demo. These add a great deal of depth and you will feel like you have different speakers once dialed in. They handle the higher frequencies with extreme control so don't be afraid to cross these over at 80hz. The sub also hides well at these higher crossover points. I think you would be impressed with the F113. The Velodyne DD18 gets my vote as well but the JL fit my area.

I have never been able to listen to any bookshelf speakers with woofers less than 5" on their own and always use a subwoofer to fill up the low registers. Yes, a subwoofer will make a huge difference to the overall sound especially if your speakers are small monitors or bookshelves with small drivers. Just make sure the subwoofer is of certain quality or standard and the setting-up is done correctly and you would be in for a treat.

I have tried running my mains (JM Lab 936) in full range with the JLA f113's. It did not work out as well. It seems that taking the strain off the amps opens things up in the upper mids and highs and these subs do a better job from 80 on down. More depth and very full.