Hi,
Up until recently I had a PM7200KI and thought it was a great sounding amp. I originally used it with some Kef Q1 speakers but not long ago upgraded to MA RX6's. Although the amp sounded lovely with them it was slightly too warm for my taste and so I decided to sell the Marantz.
I've not heard the PM6004 so can't comment but if you like the (slightly) warmer side of neutral then I don't thinkyou can go wrong with the PM7200
Incidently when I originally bought the amp they sent me the non KI version by mistake and, to be honest the difference between the two was very small so I think it is better value as the KI version seems to be going for a lot of money at the moment.
The amp had no problem driving the RX6's so I think it would be fine if you were to upgrade your speakers.
Hope this helps
Regards
Steve