I can't help with your specific question as I only have experience with 2 channel set-ups but here is some food for thought.
I have 2 separate stereos on different floors of my house connected together using the tape in/out jacks. Select tape monitor on one reciever and hear the music from the other. I purchased 50' rca cables from ebay and have never had a problem.
This allows the stereo room to be at a ridiculous volume while the rest of the house hears the same music at a more comfortable level - as opposed to wiring the "B" speakers of the same amp in a different room and having volume controlled from one amp.
Also your reciever in the garage will need amplifer inputs to work with your preamp outputs in the house. My concern would be noise being picked up if the run is too long.
This will be of no help to you but FYI with balanced ins/outs on the pre+amp you can get away with extremely long runs of cable.
cheers
