tattoo
some people who have answered you here seem to have only read your title line, then didn’t read what you actually are asking -- which is these titled tubes used in an ARC Ref 6 linestage...
the 6550 position in the Ref 6 is in the rectifier section, it is NOT used as a power tube in the signal chain at all, so its impact on the sound of the unit will be somewhere between minimal and none...
so I would not worry so much about using these ungodly expensive NOS USA 6550’s from the 1950's and 1960's and stick to what ARC suggests to use in terms of modern day produced 6550’s for the specific purpose in your unit
you may want to direct this query to greg christensen at arc (technical / service manager) -- ask him whether they have done usage and listening tests using old stock american 6550s in the Ref 6 rectifier circuit and what they have found
this is one of the hazards of posting a question like this on a forum like this... you give people license to wax lyrical on stuff they have feelings about, but they are not answering you on your specific application -- details matter!
good luck
