I have the Ultra bookshelf speakers with a pair of Hsu VTF-3 MK5's. I'm with Marshall. If money were no object, of course I'd take the towers. They look nice and who wouldn't want more drivers? I'm a little more budget conscious though and went with Ultra bookshelf speakers instead of the Prime towers (~$1000 budget).
Obviously I don't have the towers for reference, but I don't feel like I'm missing anything. These bookshelf speakers can handle 80hz and up just fine and I'm hitting the spl I like (more than enough if I want to continue hearing). The Hsu's take over everything below 80hz quite nicely. I feel like I could close my eyes and hear towers. That's basically what I have, in separates. They have no trouble filling my large-ish room.
They can handle some power and sound amazing even at high volume. This setup is by far the best I've had, and it's good enough that I think I even kicked the upgrade bug (for speakers, I'm not delusional
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). Bottom line, I love my Ultras.