I own a Marantz sr6013, and have done exactly what you are trying to accomplish. The Marantz is flaky about pre-outs. Remove the speakers, plug into pre-out, reboot several times. The signal should kick on. Then add each other pair, front, rear, etc. I decided to have 2 different amps play through my front rp280fa, I have an older onkyo a-8048 it is integrated. I use a switch. I also have a behringer a-500, I use to power a sub, it needs a preamp to boost the signal from the rocketfish wireless sub connection. I have both amps playing to subs, older amp required a small rca gain/freq regulator. There are a lot of boxes and wires, but the music from the separate type amp is better. So I get to have the system, the way I like it.