If you have your speakers set to large, they are running full range and attempting to output all frequencies coming from your source. Nothing that says you can't do this, but the situation you describe is often the result. Try setting all your speakers to small and adjusting the crossover. 80 Hz is a good place to start, but you may find a lower setting sounds better. Since you mentioned you want them to output very little bass, something closer to 50-60 Hz might do the trick. Try it out and see if that works for you. Speaker placement could be an issue as well. How close are your towers to the back and side walls? Pulling them out a bit might help as well.