will adding a subwoofer improve overall audio

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?