I’ve found towers (sic) to be more dynamic above the XO than BS. The multiple/larger drivers in quality towers share thermal and power loads while(ime) delivering much more powerful midrange performance.
So imo towers aren’t just for extension.
You point sounds very plausible, but do you have the means to measure to confirm that your bookshelf speakers fall short for "dynamic" at the levels you listen?
As the "Devil's advocate" I might think you could just as likely be hearing some resonance from the longer panels of the tower's cabinet, resulting in a fuller bass sound - which, IME, is enjoyable in many places, but a problem when the balance between frequencies is critical.
I am, however, not sure what you mean by dynamic. Are you saying simple that they won't play as loud (which I agree with), or are you saying they are more responsive (like will produce a better impulse response)?