Audio Research amp's retubing experiences

Dbarger, were your replacement SEDs in the Rew/White/Black Winged C box?

When I owned my Ref 600 Mk lll's I retubed with KT 88's which produced much sweeter midrange but at the expense of bottom end. Nonetheless, much better result than the 6550's provided by ARC

the tubes in my 600 are svetlana C 6550. since they are very good tubes, I would try to change the 6922 from sovtek, what I know a good and rliable tub, but absolutely not camparable with nos telefunken etc. A technician said me the always first tubes makes the sound....
Now I am considering varous tubes brand's , since the price for 24 matched telefunken, about 100 Euro/one here in italy is not a joke. Some good ideas? someone tested Amperex BB, Philips, Siemes Mullard's?
The best dealer for tubes in the States?
Thaank you very much...

For NOS, I have had good luck with Vintage Tube Services and Brent Jesse.

Anyone tried the BLACK SABLE SEDs 6550C instead of the regular ones ?

Tube Depot carries them at a 100% permium.