One more thing to add - if you are going with a sub - you need to have speakers that will have a flat response up to the cross over point. If you set your cross over at 80 Htz (the THX recommended frequency where you can not place the sound coming from the sub) - then all of the signal 80 Htz and below will be routed to your sub. If your speakers are too small to not handle this cross over, than you will have a "hole" in your system sound where frequencies in the 80 to XXX (speaker cross over point) will be diminished (the bose little cube speaker effect). So you need to make sure your speakers can handle 80 Htz with a flat response - and many of the better made bookshelf speakers can easily do this. If you spend extra money on tower speakers that can go down to 40 Htz - they still will not be able to handle the bass as well as a sub -- so it is kind of a waist of money as they will only be outputting down to 80 Htz anyway.
The other thing that towers buy you is speaker placement - getting the tweeter at ear level. This can easily be accomplished by getting a pair of speaker stands at the correct height for your bookshelf speakers. I have a system with front towers and rears that are bookshelf speakers on stands with a good sub - and feel I would have been better off getting that same pair of bookshelf rear speakers and having all corners be exactly the same.
Edited by Martino, July 13 2013 - 06:13 PM.