You can find these tubes on E-bay. I had a perfectly matched pair shipped directly from the Shuguang factory. They are known as the Nature Series. Tell them that Cary aka WS_trader sent you. They will consider you an adept critical listener.
Be patient, brass instruments start off a little harsh, but mine mellowed out after about 75 hours of break-in time. The proper way to break-in any component is to play a variety of music through it.
As a addicted tube roller, I can tell you that the Raytheon CK-5694s are simply the best 6SN7 GTB variant ever produced.
They are electronically similar to a European ECC32/CV-181,
but this time the American tubes are clearly superior to the European tubes in every single way. I found the Mullards lacking in dynamics with a bloated mid-range, and not much to offer in the bottom end. Fortunately they are relatively easy to re-sell to the uninitiated.
With the Raytheons, it is definitely be careful what you wish for. They reproduce everything on the recording, both good and bad. If you like digital effects, stay away from these tubes. Use a piano passage to be the envy of your friends. These tubes summon you to themselves, once heard, after you convince yourself that you are really hearing what your ears are telling you are hearing, and just as important, not hearing what you don't want to be hearing, you will be addicted. I maintain an interested buyers list and can have both the Shuguang Nature Series 845s and the Raytheon CK-5694s broken in at the same time.
Why did the NOS Raytheons go undiscovered for so long?
They have a non-standard octal pin out, which means using a small and simple adapter, or require a re-wiring of the octal sockets that take these. My 5694s are the first tubes to motivated me to re-wire my sockets. It is also a perfect time to upgrade your sockets. Stay away from new sockets, you want them made out of natural materials, with as little metal as possible.
Happy rolling!