A couple questions:
I have a Sony XBR-75X85e, an Abox A1 Max (S905X) and Mecool M8S Pro+ (S905X) running Corelec 8.90.2. All are connected through a Denon AVR-X4200H.
From fresh install my Abox A1 Max defaults to 1080P resolution and 24p refresh. To setup UHD I changed resolution to 4096 x 2160 and once that is accepted change it to 3840 x 2160. It will not accept 3840 first. Default refresh is 24 and cannot be changed. I set Adjust display refresh rate to “Always”.
My Mecool M8S Pro+ defaults to 1080P but can be changed directly to 3840 x 2160. Default refresh rate is 60 and is set to always.
Just curious as to why 2 boxes with similar hardware would have different settings and why the Abox will not accept 3840 without being changed to 4096 first. Help is appreciated.
The abox has a S905W, not S905X, which doesn’t support 4K@60Hz, AFAIK.
It could also be a matter of input settings on the AVR/TV, whether HDMI 2.0 (Deep color?) is enabled or not.
GUI resolution should be left at 1080p, Kodi will automatically switch to 4k when you are playing 4k content, refresh rate should also not be locked to 60hz as this will cause playback issues with some content.
In case of GUI resolution at 1080p and SD content on 4k TV you get double interpolation (SD-HD by the box and HD-UHD by TV) with worse quality compared with GUI at 4k
With which box? If playing SD content the box will switch to correct resolution. I always set GUI to 1080p 60Htz, never had a problem, regardless of box in play 
I’d be suspecting the TV
In my case it’s Mini M8S II (S905X), but I think it doesn’t matter. SD means 576p (PAL) and 480p (NTSC) and there are no such resolutions on TVs. That is why I prefer GUI at 3840 x 2160 and interpolation by the box
@alex Those resolutions are available on all of my (4) TV’s and GUI, at the resolution you ‘prefer’, is not advisable (as you have been advised). Most likely a TV (or unsupported) issue
I thought this was a S905X box. The original add didn’t identify it, but someone confirmed it in the “questions” section. Guess it was BS. I haven’t seen any viewing issues at 4K. Been watching the hockey playoffs on a 4K channel and it seems fine. Is the refresh rate not auto-switching based on the source? My Mecool shows 60 after setup.
Also, I can confirm that a different box, an Abox A2 which is S905X i can set it properly at 3840 x 2160 and 60hz
Was the consensus here that the box should be left at default resolution 1080p and 60 Hz and allow KODI to choose resolution?
The TV upgrades the incoming picture (4K X-Reality PRO™ upscales images to near 4K clarity and HDR. Our newly developed 4K HDR Processor X1 outperforms our original 4K Processor X1 with more real-time image processing power. It takes even non-HDR content to near 4K HDR quality).
that has always been the recommendation, gui should be set to 1080p and kodi will auto switch to 4k if needed.
Is auto switching configurable?
Does kodi switch to lower resolution (e.g. gui at 1080p and content at 720p)?
It will be once CE moves to a newer Kodi base. The way resolution and refresh rate switching is handled has changed in newer Leia builds, it now uses a whitelist. With this whitelist, “Change refresh rate” should be set to “Always”.
right now I believe automatic downscale is not supported
All of my TV’s report they are switching up/down resolution/refresh rate as expected, why do you suspect otherwise?
Automatic downscale will be available in next CE release if you choose correct resolutions in whitelist settings.
So are my TV’s not actually switching at the moment?
I used wrong downscale definition. I mean lower resolution, for 720P video for example. If you have 720p25Hz video and your TV is set to 1080p 50hz than you see it as 1080p25Hz or 1080p50Hz on your TV.
With whitelist it possible to see this video in 720p resolution also and use TV upscale algorithms
Whitelist Kodi PR
Oh, OK. I never even noticed it had stopped changing the resolution (and there’s a big pop-up on the screen when changes happen) 
downscale have never been supported before, I remember kzsaq saying that on the le forums.
e.g. kodi gui resolution set at 1080p, you play a 720p file, the tv resolution will not switch to 720p.
but it’s good to hear that it will now be supported on future versions.
Really good news to the inmediate future of CE!!!