Cary DMS 500/550/600 Feedback Needed

I'm looking to upgrade to a DAC/Server and the Cary DMS has pretty much what I need. I plan on using its WiFi for iOS/Cary app support and using a USB HDD as a file source.  Adding an Ethernet cable and poss. NAS later if needed.
I won't be able to hear one before purchase and I'm aiming for one of the used DMS 500's Cary sells direct.

I'm looking for owner feedback/experience with the early or later models. I have seen a few unhappy experiences but I suppose that is somewhat expected.  My main question is whether the 500 is a good unit to commit to or would a newer 550/600 be a better bet?

The other alternative is Lumin.