Mkilpi. I've been wondering the same thing. Hopefully you'll get some response. Also would like to know if the NOS 6H30's make a difference.
Interesting subject. Any thought on Frank's question on the 6H30 tube regarding the audible sonic differences of manufacturer/dating of the tubes? Also applicable to the 6550C?
I need to replace tubes later this year. Would be very interesting to know if a potential audible improvement on the already great (glorious) performance of the ref3 is possible!
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