Thanks Guys, I appreciate the help. I contacted Marantz about a year ago, when I still had the NA7004 but they could only suggest the same things - resets, firmware upgrades etc. I got to the point with the NA7004 that I wasn't playing music through it because it was such a pain to use. I switched to Cambridge Audio for the streamer and it works like a dream both in terms of sound but also useability.
That's where things aren't quite so easy, though, with the bedroom box because I use both speaker outputs on the MCR - one pair for the bedroom and one for the bathroom - can't beat a soak to your favourite music. So while the obvious thing to do would be to go for the CA Minx XI, I'd really like to stay with the Marantz for that reason.
What I'm thinking is that the streaming module is likely to be very similar in Denon and Marantz kit of any given generation so the MCR603 and the NA7004 probably had the same module. In our house, we also have a 2013 Denon Cocoon in one of the kid's room and a 2014 Denon AVR in the sitting room which both stream from the NAS faultlessly so I reckon it's probably worth risking the new MCR611 for Xmas.
I'm surprised no-on with a 610 or 611 has commented. Given the good experience everyone else seems to have with the 603, I should get a reasonable price for it on ebay.