How to hook up a bass speaker on stereo system

I am using the san sui 9090 db which has A, B, C, A&B, A&C speaker outs. I have Two eight ohm speakers playing through the A outs.
What I would like to do is hook up a 15" speaker to the B speaker outs. It is not powered and is a raw component that has been boxed. Is there a way to simply hook it to the reciever and not jeopardize/ create an ohm mismatch with the reciever? If I wanted to dedicate this speaker as a sub and run to mids and highs on the A outs, what would be the best way to acomplish that? The san sui has pre outs. Also I have two san sui 9090's and wondered if the sub could be dedicated to one amp and possibly eliminate the need for another amp and electronic crossover? If it is possible to simply hook the raw bass speaker up and I wanted to put a passive crossover at the speaker to cut out mids and highs can this be done and how would I know size of _______ to put there? Trying to create a nice sound with limited financial resources...