I use Synergistic Research Black fuses in my Rex Monoblocks, Rex preamp and I love them. I think they are worth the relatively modest investment.
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batman1971 - I bought my VK-55 used, it's my first tube amp, and it came with the standard output tubes, Russian 6C33C-Bs. The driver tubes were Sylvania Chrome Domes, now I am trying Upscale Audio's "Early Russian" 6H8C/6SN7s. Initially these did not sound too good - mids were front and center at the expense of everything else, which faded into the background. After ~40 hours of break-in, they started to sound very similar to the Chrome Domes; after 100 hours they are very pleasing to me, mids have more resonance and highs are slightly clearer, mid bass seems to be a bit more powerful as well. Would this be described as "more tubey?" Open to trying others...any recommendations? Also, I have thought of finding a second and going mono, is there a downside? Worried about more distortion.