I agree that in general, the answer to this question is going to be tube specific. Take for instance the famed 12AT7. It is the general wisdom that the Telefunken NOS ECC801s is the best of the best. At $350+ per pair, they better be! But I have been blown away by the sound of a humble pair of Chinese (Penta Labs made by Shuguang) 12AT7s ($30/pr) and by Mullard 6201 gold pins ($150/pr). I am not crazy about the JJ Teslas (a bit bright and grainy). Of equal importance to the tube itself, is the selection stringency of the tubes. The 12AT7 is used as the first gain stage of my VTL Siegfrieds and triode matching, low noise and low microphonics are critical. I highly recommend Doug's tubes (http://www.dougstubes.com) for his superb personalized service in this regard.
As far as ouptut tubes, I have been around the block on this every way from Sunday. First, let me state unequivocally that I am not a fan of 6550s as I believe the KT88 is simply a far more musical tube. 6550s have an inherent grain structure in the upper midrange that is rather characteristic of the tube style regardless of manufacturer. Having long ago substituted KT88s in my ARC Ref 300 MkII amp for the stock 6550s, I strongly believe that an ARC owner who has not tried KT88s in his amp doesn't know the true potential of their amplifiers. Unfortunately, ARC does not support KT88s (unlike VTL) so you have to do it at your own risk. Fear not, the bias is identical and the switch should be seamless. Regarding KT88s I definitely not a fan of the famed SEDs from Russia. Once again, IMHO the Shuguang factory is making the best tube of this type on the planet. These are available in several iterations and bottle shapes. However my search has ended with the beautiful Shuguang 88-98 which is available from several sources. This is also identical to the Valve Art KT100 and the PentaLabs KT88-98, both of which are OEM'd by Shuguang for these labels. The Shuggies are spectacular tubes and puts the Russians KT88s to shame. The Valve Art KT100s have been running in my VTL Siegfrieds for some time now and are simply fabulous.
As far as ouptut tubes, I have been around the block on this every way from Sunday. First, let me state unequivocally that I am not a fan of 6550s as I believe the KT88 is simply a far more musical tube. 6550s have an inherent grain structure in the upper midrange that is rather characteristic of the tube style regardless of manufacturer. Having long ago substituted KT88s in my ARC Ref 300 MkII amp for the stock 6550s, I strongly believe that an ARC owner who has not tried KT88s in his amp doesn't know the true potential of their amplifiers. Unfortunately, ARC does not support KT88s (unlike VTL) so you have to do it at your own risk. Fear not, the bias is identical and the switch should be seamless. Regarding KT88s I definitely not a fan of the famed SEDs from Russia. Once again, IMHO the Shuguang factory is making the best tube of this type on the planet. These are available in several iterations and bottle shapes. However my search has ended with the beautiful Shuguang 88-98 which is available from several sources. This is also identical to the Valve Art KT100 and the PentaLabs KT88-98, both of which are OEM'd by Shuguang for these labels. The Shuggies are spectacular tubes and puts the Russians KT88s to shame. The Valve Art KT100s have been running in my VTL Siegfrieds for some time now and are simply fabulous.