Dennis answered my email and suggested trying another tube in the 6bx7 position. I put in a 5691 tube I had (6sl7 variant), and initial listens allow me to think that the 6bx7 tube was the problem. I'm getting much quieter operation with the 5691. Will continue to listen and report back, but I'm thinking that tube was the problem. If it was, I guess I'll need to figure out whether to buy another 6bx7 (they're cheap) or continue to use my 5691 (not cheap!).
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