Though I'm still waiting on the 5751s I ordered, and the new rectifier tubes, I just tossed in a JJ / Tesla 12AX7 for the input, and two Ei Elite Gold 12AX7s for the outputs, and...wow. Just...WOW.
What a world of difference relative to the very worn out Chinese tubes with which the Symphonies came. I wish I'd've gotten these new tubes before the Chinese tubes wore out, so I could offer a better comparison. But replacing the worn out tubes results in thicker, more robust bass, better extension; mids have regained their richness / detail; highs are coming through clearly and crisply instead of the previously dull, flat sound from before; and the soundstage has opened up nicely. I'm kicking myself for not replacing the old, tired tubes earlier. Anyone out there who's been delinquent in replacing their old tubes, swap them out NOW! Can't wait to roll different tube combos. This is gonna be fun!