Why?
What do you expect towers to give you that bookshelves don't? More bass? How much do you expect to pay for your towers one day?
Robert, let me expand on this a bit...
I'm using these for side and rear surrounds...
They cost $1k each, and received this award:
Their frequency response is: (+/- 3dB) 45-20,000 Hz
Now look at these:
They cost $195 a pair.
Their frequency response is: 48 Hz - 20kHz (+/- 3db)
The difference is about half a note. Neither is low enough for the gut shaking rumble of thunder or earthquakes or monster stomps. A sub woofer is still needed.
Thus my question about what you expect from towers and how much you expect to spend. Clearly, $1k each is not enough to preclude the need for a sub.
If I knew then what I know now, I would have bought 4 of the bookshelves for $400 and saved myself $3600.