why use towers if you've got a sub??

It can be difficult to integrate a sub. But towers eliminate the issue.

Yes, I agree with the above poster... I would also not cut off anything below 80Hz from going to your monitors unless they were not designed to reproduce such frequencies. Subs don't do a good job of reproducing mid bass frequencies, and it is difficult to get them to match the tone, attack, and speed of your main speakers. The quality of bass that different subs produce can vary a great deal. Some subs can help the music in certain respects, but other times they can take away from the experience as well.

Well, this is one area I am in the minority, but I like big speakers for my front L/R. I've noticed that in a lot of action movies there is prodigious deep bass in the front channels, and that, contrary to the conventional wisdom about bass being non-directional, I can clearly hear deep roaring sounds and percussive effects coming from the front left or right side. Now, I do realize that the deep LFE rumble sounds are better produced by a sub, but I don't really like a whole lot of that rumble anyway.