Final update:
Fuchs Audio could not find anything wrong with the Cayin HA-1A MK2. Using the stock tubes they said:
I have the amp back now, but the hum and static is even worse. I have tried turning off all electronics around the amp, all electronics in the same room as it, and switching rooms to test it. Static and hum remain in all cases. To make it more interesting, I picked up a Feliks Elise MK2 and have the Elise in the same location where the Cayin was in my room. I am using the same power cable and RCA cables, and there is 0 hum, hiss, or buzz—absolutely pitch black. At this point, I am out of ideas and it is obvious that the Cayin amp is not viable in my current setup.
I am utterly confused as to how one tube amp can operate perfectly in the same environment and conditions while another is an absolute nightmare. I don't understand how one repair place can tell me the amp is working perfectly while another says the opposite. I want to love the Cayin amplifier so badly. It is a beautiful unit. The variable output impedance is amazing! You can go from a warmer solid-state sound to tubey goodness with a flick of the dial. The 12AU7 tubes used in the output stage are easily found, can be cheap (although
nicer 12Au7s should definitely be considered), and affect the sound in such a way that tube rolling is possible, impactful, and most importantly FUN! The Cayin would have been perfect for me but whatever is going on with my unit makes it impossible to use and enjoy. I would like to sell the Cayin to get back some of the money that I spent on it, but I can not promise any buyer that they will have a good experience with
THIS PARTICULAR unit. If they are lucky, the amp will be perfect just as Fuchs Audio described. If unlucky, they will have an experience similar to mine, be disappointed in their purchase, and efficiently have wasted a large amount of money.
In closing, I would like to thank
@Andykong, the seller that I purchased the Cayin amp from, and Rich from Fuches Audio. I have bothered you all quite a bit about this amp and the problems that I have had with it, and I appreciate you both taking the time to respond to me, offer suggestions, and work toward a solution. Although this is a disappointing resolution, I sincerely appreciate all the time and effort that you all have spent trying to help me. Thank you.