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

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.

My sub is a JL Audio f112 and about a year ago I went from Focal Micro Utopia BE Monitors to Avalon Ascendants. I now find that I’m listening to the system instead of listening to the monitors… then the sub… then the monitors… then the sub… … … and wondering.

Interesting the way we all think and hear differently about this. I find a sub to be near essential for music, even with near full range tower speakers. I don't give a hoot for home theater applications, so it's strictly about music for me. Perhaps listening environments vary so much that that is a major factor in our differing opinions? I've used a sub with Maggie 1.6's, my ACI Talisman SE's had built in self powered subs, and I ended up adding one JL Fathom F112 to augment the 32hz. extension of my Tidal Piano Cera's. I cross it over a 40hz. and it is subtle in the way it affects most music. But I love the way it adds ambient cues to music. Small things like hearing the rumble of the stage under the orchestra interact with the whack of a tympani add immeasurably to the sense of suspended reality a good system induces.