The Marantz is fully capable of both 4K passthrough and 4K upscaling according to Marantz. As for power, I have no concerns at all. I recently purchased one and have it running a mixed bag of speakers and let me tell you it sounds awesome. When I say mixed bag, I mean
2x B&W 602 S1 Front LR 8 ohms
1x B&W LCR60 S3 8 Ohms
2x Pioneer BS-22 6 ohms
So as you can see I'm not using the most sensitive speakers and in fact I'm mixing impedance ratings. The 7008 Has no problem at all handling them. In fact, it sounds awesome. Far better than my previous HK AVR-3600. If you want the Marantz, I say go for it.