Last, there is debate on tube rolling. Is this the same as sighted bias/cables? Or are there differences at different bias or volume?
I'm bummed because I had a chance to actually blind test tube rolling in my amps, but it didn't happen. For a while I had two pairs of the same tube amps.
I own Conrad Johnson Premier 12 monoblocks, which are quite old, and I bought a more updated pristine-condition pair at a price below what I knew I could sell them for. A "bonus" was that the pair had gone to Conrad Johnson where then went over it and also did the Teflon Cap "upgrade" - one of their expensive upgrades (yes, I know how that will be greeted here). So I originally planned on keeping those newer, better condition amps and selling mine. I wasn't necessarily expecting a sonic difference - it being more of a chance to have a newer, better condition pair of Premier 12s.
As I mentioned in some other thread, I was surprised that I seemed to hear a very "obvious" difference - the bass seemed tighter and deeper, the highs a bit brighter, and everything sounded "bigger" than what I'd been used to. The guy who sold it to me sent it with KT120 tubes (and different rectifier tubes) where I'd always had 6550s in my amps. So I swapped those KT102s to my old pair of CJs just to check and much of what I "heard" transferred to my old amps.
That was so fun I tried all sorts of tube rolling, and often perceived differences. Even more surprising, the new CJs (with the Teflon caps) consistently seemed to have tighter, more controlled bass even when using the exact same tube compliment from my old amps.
Naturally I wondered how much could be due to human bias effects and would have loved to do a blind shoot out between my old and new CJs, both for tube rolling and to see if I could actually identify the "updated" model. Perfect situation! I even asked about this in ASR because I was aware it's a bad idea to swap speaker cables between live tube amps. I got some suggestions as to how to protect the amps/speakers but in the end chickened out. My finances demanded I had to sell the new amps pronto! And there was no way I was going to even chance something happening to those amps (or my speakers) at that point. Someone with electronics knowledge/experience I'm sure would have had no problem going ahead, but I just wasn't confident enough. Sold them shortly after.
I still wish I could have done that blind shoot-out. (I failed my ASR brothers....)