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

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.

You dont! If your main speakers are good down to 34-44hz it covers every instrument except organs and synth. Subs are for home theatre boom booms, so leave them alone for real music.

Subs are for HT, I use my towers for music.

There are HT subs and there are music subs. Large speakers create more room problems than small speakers not to mention large floor standing speakers themselves are big reflective surfaces. If you think it is difficult integrating a sub then you have never owned a REL. REL subwoofers will integrate with any speaker from monitors to Quads. Monitors speakers with a sub also give you the flexability of pulling the monitors into the room for a more spacious sound while not sacrificing bass performance. If you are a Vandersteen 1, 2 or 3 owner there are other benefits to using a Vandersteen sub because of how the Vandersteen sub connects to the system. With those systems a Vandersteen subwoofer gives you advantages in increased performance that is hard to imagine.