I thought that I would weigh in on this.
I've had a 550 and a 390, the latter with a power amp but I did listen to the unit itself, so I feel that I can answer.
It has to be said that Arcam are to be applauded on keeping an old platform relevant with releases that allow Dolby vision pass through and IMAX compatibility etc. Whereas Yamaha release yearly models with near no change Arcam has stuck with their platform licenced from, or at least Co developed with, Audio Control
That said, the Arcam line is way to quirky for me and I'm very handy with hardware and networks. All too often there will be something that cropped up which made me have to do something to fix it. Bass management not well implemented, hissing through the speakers due to high level trims. Dirac crashing and corrupting saves (though v2 is better) only one Dirac slot making changes difficult to compare, DLNa is broken and every once and a while things just simply wouldn't work (normally HDMI or network related) and the unit needed a hard power cycle.
Arcam units do sound good but not amazing. Personally I'd wait if you really want an Arcam. I found the Arcam to be almost unfinished compared to the smooth operation of the Japanese competition and it feels like Harman/Kardon (or Samsung that bought Harman that bought Arcam) are milking the platform for as much as folk will pay to shift stock - the Lexicon receivers are exactly the same and even more expensive. With NAD and Anthem releasing product later in the year Arcam would be wise to match them. Speaking of Anthem...
...get an Anthem 720 which is a far more complete package compared to the Arcam's. I have also had a 720 and regret selling - I've never regretted selling the Arcam 550 or sending the 390 back. I would also say that the Anthem never locked up, is really simple to use, never had any software issues with ARC and for me ARC is, if not technically, then at least sonically more appealing vs Dirac. Anthem amps feel stronger driving difficult speakers, Linn Majik at the time, and with a new ARC releasing shortly it would be my tip for money spent.
Neither Anthem or Arcam work as well as a Yamaha/Marantz and neither of them have as many useful features like Musicast. If you want something to just work and handle pretty much anything, buy Japanese. Also, if you are interested in future compatibility, then a Denon 8500 with it's future HDMI upgrade may be a better bet than any of them but it won't sound as good - it's something you'll have to decide.
NAD is another option but I've not heard them and they have had their issues. I believe that most are fixed with NAD offering pretty good customer service. NAD also appears to have implemented DIRAC better than Arcam. I wouldn't pay full price as I have read that NAD are to release a new platform in the Autumn. If there's a Sevenoaks near you, you could have a listen?
Russell