I have never heard the PM6004/CD6004 combination with the RX6 sop can't help there, but I have been using (up to recently) a Marantz PM7200KI amp witth a DR6000 cd recorder (so at least the same manufacturer lol!).
I think that the Marantz / RX6 combination sounds great and despite the speakers being fairly close to a rear wall, I don't have a problem with overblown bass.
I guess that my amp is slightly warmer sounding than the PM6004 so I would have thought it would work pretty well with the MA's. I have never really got on with the NAD sound as it always seemed bass heavy to me so can't comment on the combination but I do think it would be easy to overdo the bass side of things on the RX6 if you use electronics that are biased in that direction.
I do think though that the RX6 deserves a better amp than either of the Marantz amps mentioned. My amp has actually died so I am looking for a replacement. It has been suggested to me that the Naim XS would not be overkill considering the source I use. I'm still just getting ideas at the moment though.
If you decide to try it I would be interested in your comments.
Hope this helps (a little)