If you want to stick with Onkyo, take a look at the TX-SR707 as it has pre amp outs to each channel and will do the job.

http://bountii.com/product-3649-onky...FcEz5wodMBW7aA

However, the only benefit that I can see in using the Marantz amps is for 2 channel music. What is the rest of your gear like?