I recently rolled some tubes in my '69 Twin, comparing both tubes, and bias settings. I recorded each permutation using a clean Strat, and compared in a DAW. Tube sets: Winged C, well-matched NOS RCA black-plates, TAD short bottles, and a few others, even a set of JJs (yuck), and Sovtek 5881WXT. The Winged C set topped everything else, with ease. I like them biased cool, at 45% max dissipation, or around 34 mA. Some really pretty tubes. Very open, airy, and musical. Also, the initial quad purchased from thetubestore wasn't as well-matched as I wanted, and customer service was excellent.