will adding a subwoofer improve overall audio

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.