Point taken!
That is Marantz being overly conservative, then relaxing their specifications for their X009 models to better match the competition. Given how sloppy speaker manufacturers are with their "nominal impedance" specifications, you might understand their reluctance to spec a 4 ohm nominal speaker which could dip below 2 ohms and still be 4 ohms nominal.
As a personal observation, my old SR-6001 was rated as minimum of 6 Ohms, but when benched, it was discovered to have the exact same amp section as the 7001 and produced 134WRMS @ 8 ohms and 204WRMS @ 4 ohms for two channels. Moreover, it produced 90WRMS @ 8 ohms and 130WRMS @ 4 ohms into all five channels!
The 5008, 6008, and 7008 were all specified as a minimum of 6 ohms driven, and the next year the 5009, 6009, and 7009 were all rated at 4 ohms minimum load. Yet Marantz makes no mention of a new or improved amp section in their marketing materials (you can bet they would if they could).
The 7008 was Marantz's flagship amp, and I would surmise it is as capable as my 6001 and a currently produced 7012 with 4 ohm speakers.
But Damn, due diligence and all, you are making me do research...
S&V did testing of the 6 ohm minimum SR5003 and the 4 ohm minimum 5009!
S&V is not real thorough about addressing 4 ohms but the lab reports both have MJP's initials on them so I feel they are comparable data sources.
For the "6 ohm" 5003:
For the "4 ohm" 5009:
These are not full range RMS watts, but it is clear that the 6 ohm rated 5003 is more capable at 4 ohms than the 5009 which is rated for 4 ohms!