I had the same experience as the OP. The email response from Upscale was:
"We received your order, but unfortunately we cannot fill it. The SED 6550C used to be the best tube for your application, however the last batch of tubes to come out of the SED factory before it was shut down, have had issues with a soft vacuum. In products like ARC, Rogue, VTL, and several others, this causes the tube to fail prematurely.
We recommend, as does Audio Research, the Sovtek 6550WE..."
I was lucky in that I have a few extras so was able to use one of those.
"We received your order, but unfortunately we cannot fill it. The SED 6550C used to be the best tube for your application, however the last batch of tubes to come out of the SED factory before it was shut down, have had issues with a soft vacuum. In products like ARC, Rogue, VTL, and several others, this causes the tube to fail prematurely.
We recommend, as does Audio Research, the Sovtek 6550WE..."
I was lucky in that I have a few extras so was able to use one of those.