If you can find that link, please share. As far as I know, in terms of the 3800 vs 4800, the most informative and reliable source is the video I linked in post#3. It was presented by a product manager of Masimo, owner of the Marantz and Denon brands. All other (numerous) Youtuber presented videos simply regurgitate the marketing info you can find from the Denon websites and some online reviewers including Audioholics. The Masimo training video actually dived into the internal parts and block diagrams, many of which are only found in service manuals.
Another reliable source is the one linked below, by Simple Home Cinema:
Denon X3800H versus Denon X4800H – Simple Home Cinema
Simple Home Cinema also made reference to, with link included, that same Maximo training video mentioned above.
If you are not particularly into the style of the Cinema 50, and if you are in the US, or Canada, then I would highly recommend the X4800H because in the US it is actually $1 less than the Cinema 50 that is really an AVR-X3800H with lipstick (borrowed from you lol..) In Europe and Asia, the price between the X3800H and Cinema 50 are much closer, about 200 Euro, so there isn't much saving going with the Denon.
By the way, I am not surprised your local dealer won't set up your target devices for a good AB comparison, but you tell them you will buy one of the 3 after the comparison then they may do it. It is a lot of work to set up 3 or even 2 for a real AB comparison that allows you to switch between them quickly. Even if you can do it, it will not mean anything unless it is level matched to within 0.5 dB and following the DBT protocol.
Are you still concerned about "sound quality" difference between the ones you are considering? If so, don't, I have compare my own avrs, avps to my very good quality, much more expensive "separates", for the most parts there were no audible difference and measurements would support that too. Those who heard differences are most likely due to other factors. I would still pay more for the look/style, better build quality and convenient, useful features.