Another Old Guy here. I can definitely appreciate your feelings on this matter. I checked Rotel’s website (as I am sure you have already done) and unfortunately, it does require a copy of the original receipt for warranty work. However, IMO your online registration SHOULD have been enough.
It seems as though more and more companies have gotten away "Customer Service" in more ways than one. I am sure many of you other old guys can remember when "customer service" used to really mean something. I will use Craftsman tools as an example. You paid a slightly higher premium for their brand, but there was no question if you needed to replace a tool from them.
Before "planned obsolescence" became an "industry standard", companies used to actually compete to make a better product than "da udder guy". In addition, they knew that by building a better product, the customer developed "brand loyalty" as well.
(my parents owned an old Norge refrigerator that lasted 25 years before my mom made my dad buy her another one because the "color" was not in fashion.)
Maybe that is why companies like McIntosh, Marantz, and others are still in business?
And, perhaps that is why brands like Adcom are still selling today because of their reputation of longevity? If I was in your shoes, I would find the address/phone # of Rotel and try to contact a supervisor to explain your situation. If you do not have any luck there, I would continue to make your concerns known on social media. Sometimes, as they used to say, "the squeaky wheel gets the grease".
Best regards,
Rob