ARC Ref 3: Tung-Sol 6550 in power supply?

I thought that I would resurrect this topic perhaps with a little more pertinence to the later Ref 5SE and in particular with regard to comment made by Ethannn back in 08

" 07-13-08: Ethannnn
I would recommend the (blue label) GEC KT88 for this pre."

I would appreciate any further actual user expearience with regard to substituting 6550 with a KT88 in this circuit, given that they are very close in specification.

I did make inquiries with ARC only to received the standard

" We do not recommend , you will comply "

response .

I fully appreciate that there would be no economic incentive for this consideration by ARC , however that is not quite the same as ~

" Whilst we would not warranty support or be liable in instance of this substitution please find attached our considered opinion on a technical/circuit basis alone "

Most interesting Ptcl99 , my regards for your spirit in pursuing your experimentation, especially with your utilization of non NOS valves.