Not that I am aware of, the way I understood it, preamp mode would apply to all channels. It disconnect the preamp output to the power amp, but the power amps will still be energized, by a small bias current.
No other Denon or Marantz AVRs except the Denon flagship such as the X8500H and X7200WA have this preamp mode feature. The older AVR-4520CI may have it too but I doubt it,
@lovinthehd may know. Obviously the X6500H is not a flag ship and it does not have that feature for sure.