First thing, I am not a 4K expert. I only know that if you want to support 4K with all the new features, you need to go with one of the big company items (i.e. Sony, Marantz, Anthem). There are some boutique companies supporting 4K, like Krell, Theta, McIntosh, but they all have additional quirks and cost issues. (i.e. Krell Foundation has no Atmos, Theta is insanely expensive, McIntosh is buggy firmware with no support). I don’t know what to say about your issue other than it sounds like a firmware issue with regards to certain 4K standards. One thing you could try is getting a different HDMI cable. The higher bandwidth used for 4K can be a problem if the HDMI cable cannot charge/discharge fast enough for the digital square wave pulses at that speed. This could translate into communications between source/processor glitching and lost data. Maybe try a Wire World HDMI cable?
I have taken a quick look at the AVM 60. I have not heard it, but I would place it -probably- as equal to the Emotiva XMC-1. It is probably very nice sounding, but there are definitely better. Anthem is more of a boutique company and the AVM 60 comes in at $3k. Whereas Marantz company (D&M) has over 200 R&D engineers for their product development and has a huge reseller market. The Marantz is going to give you more per dollar than any boutique item. I think Anthem had a lot of success with their $9k D2 processor, so they took the software and put it in a very trimmed down platform so that they could sell it at a lower $3k price. My gut says that the 8802 (and possibly even the 7703) are going to sound better. But again, I have not listened to it.
Subjectively again, there are boutique processors that will beat the 8802a. The $9k Classe SSP-800 sounds better than the $4k Marantz 8802a, but Classe doesn’t support 4K, atmos, etc. And it’s $5k more expensive. The ~$6-7k Krell Foundation may or may not be better -- it’s controversial (some say it’s better, some say it’s not). Marantz 8802a is not as good as $12k McIntosh MX151, but it’s close.
There is a huge amount of positive reviews on AVM 60, but remember it’s all relative. For the money, the AVM 60 give you a whole heck of a lot. Many people love the Anthem room correction system. But like I said, it’s all relative and it’s a great product at the $3k level.
I auditioned the Emotiva XMC-1, which also had a huge amount of positive reviews (like stating that this thing is just awesome sounding). What I found was that the XMC was very very good for the money spent ($2k at the time), but it would not beat out my 15 year old Krell HTS, even when the XMC was playing hi-res through HDMI. The Emotiva had a somewhat closed-in sound (common on their preamps/dacs) and did not have the "separation-of-instruments" clarity I wanted. The XMC-1 will soon be at the $3k level because of new features (HDMI2.0/4K support, Dolby Atmos, DTX:S, Dirac Live room correction).