I have read a lot of threads over the years, decades, of people saying that a bookshelf doesn't have the performance that a tower speaker has.
Well after having both, when a bookshelf speaker is setup with the appropriate subwoofer, the bookshelf 2.1 can actually out perform a tower speaker for music.
Most tower speakers have limited bass response and output. Also today, most tower speakers max out with 6.5" woofers while a few current designs go up to an 8" woofer.
Given if you pair a large bookshelf speaker (with 6.5" woofer) with a subwoofer, you are essentially doing what the tower speaker design does. I know with my Elacs, their tower speaker upgrade only adds an additional 2 6.5" woofers that are crossovered at 90 Hz. I actually crossover over at 80Hz to my sub so the towers are essentially what I'm doing but instead of a high powered 12" woofer I have, are using little 6.5" woofers, but 4 of them obviously.
As far as presentation, soundstage and imaging, my main system gives up nothing to the upper tower models.
Now with a smaller ~4" woofer in your bookshelf speakers, you are crossing over higher which can make it harder with some subwoofers as they have fast roll off at higher bass besides the loss of sensitivity in the bookshelf speakers making them need more power to create louder output. The same could be said of some bookshelf speakers with 5-1/4" woofers as they can have low sensitivity as well but should be able to crossover to a subwoofer without much difficulty.
