will adding a subwoofer improve overall audio

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