I am using these tubes on all three of my FX-Audio Tube-03 preamps and they sound great. Tried Chinese 6J1, 6K4, Russian 6J1P-EV and 6ZH1P-EV but these tubes definitely stand out above the rest. Way above the stock tubes. Excellent bass, details, wide and deep stage, great separation, warmth. But they definitely need some burn-in (I would say 20-30 hours at least) to eliminate the distortion that can occur in the lower frequencies (bass) when they are new.