Hi Dale,
The towers are going to work great without a standalone sub. You are going to have plenty of volume and by having dual subs you eliminate placement problems. The towers play down to 30Hz so the only thing you will be missing is the lower 5-10 Hz that come from special effects in action movies. To get that, you would need a 10" or 12" sub which would probably overwhelm your room both visually and audibly. I am very sure your condo neighbors wouldn't appreciate it. With just the two towers, you are going to need to host a couple movie nights to keep them happy.
For music, the towers are going to give you all the bass you need.
Your current bookshelf speakers will probably work fine as surrounds for movies. They will have a different timbre but it probably won't be distracting in a home theater setup. If you want to listen to multichannel music, I would suggest a matched system but you can always upgrade down the road.
Cheers,
John Wanderscheid