I recently acquired a matched pair of 'holy grail' old RCA blackplate 6L6GCs. With breathless anticipation, I swapped out the 7581-A tubes, adjusted the bias, and cranked her up. An hour later the 7581-A's went back in. Clearer, quieter, louder, tighter, stronger bass, way more headroom, way more chime, way prettier all the way around. I have tried a lot of tubes. For my tastes and my money, I will be going with 7581-A's from now on.