A rack with both tne amp and pre-amp would be best. The rack doesn't have to be in the center, it could be at the side. If you can go balanced, which you can't, then a longer run of IC and a shorter run of speaker cable is best; however, a 20' run of speaker cable is not really that bad. Your cable budget is sufficient that you can afford some good cable.
When you intitially posed the question, were you going to place the amp betweent he speakers and have the pre at the other end of the room?
Since your using Rives, I'd ask them. They certainly have tons of experience in this regard. Be sure to post some pictures for us when you're done.