Hard to say for sure, but leaving the sub in should allow you to maintain higher SPL in the bass range. You could just crossover at about 60Hz. That way, your new towers will get some work below the level your current bookshelves go, but yet won't be responsible for the entire range, allowing them to put out clearer sound. Only way to tell for sure will be to try them out for yourself once you get the speakers in your system though. Try just the speaks themselves set to large (no crossover) and try them on the same material crossed over aroun 60hz with the sub and decide for yourself.