will adding a subwoofer improve overall audio

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.