How to hook up sub?

I have my main speakers attached to 5 way binding posts on my amp with spades. I use two sets of amp posts, one for the tweet/mid and one for the bass. Can I also connect to one of these sets of posts to hook up a powered sub via banana plugs? Any difference in going with the set handling the upper or lower frequencies? Any issues of impedence etc? I do not want to use the sub's crossover but rather to keep the main speakers directly hooked to the amp.
No problems that way. Use either set of terminals.

Kal is right, you should be just fine.