If all you want is more low end, a subwoofer or two will take care of that. And that would be, by far, the most economical way to get truly deep bass.
Now, if you are wanting to improve other aspects of the sound, then replacing your main speakers is a good idea.
As for stand mount versus towers,
TYPICALLY,
at the same price, towers will play louder and give you deeper bass, and stand mounts will be better in every other way.
I recommend that you go out to all of your local audio stores, and listen to as many types of speakers as you can find (e.g., ribbon, dome, horn, whatever), of as many brands as you can stand, that are within your price range, with music with which you are familiar, of all of the types of music to which you listen (making a CDR is a good idea for this). Then buy the ones you like best.
I recommend putting Magnepan on your list of speakers to audition. Since you have not told us how much you want to spend, I will recommend that you spend a couple of thousand on these:
http://www.magnepan.com/model_MG_16
If you must keep the price down, the Magnepan MMG is generally regarded as a great bargain at only $600:
http://www.magnepan.com/model_MMG
My brother has a pair of them, and they are pretty much exactly what the general consensus says about them. Look for online reviews and read them.
There is also a model in between, if you have an in between budget.