In order to bi-amp, you need 2 amps. I don't know if the Marantz is set up like the Denons where you can set the 6th and 7th channel amps to do biamping of the fronts, but if it's not, you're not biamping. And you probably will damage the receiver. It's also debatable whether biamping will add anything without using an external crossover.

A separate amp will work with all the modes.

I am the Doctor, and THIS... is my SPOON!