Guido,
I compared the stock ARC 6550C, the Tung-Sol
6550C (current production), and NOS Tung-Sol black
plate 6550C in my ARC Ref 3. I was always happy with the
stock 6550C tube. Ultimately I found the NOS tube added alittle more warmth that I really found captivating. All the fantastic resolution was there in spades but with alittle "CJ" warmth. I did not care for the current production Tung-Sol. I found it uninvolving and sterile
sounding.
Vmk