It is possible that the tube gear in question could be tough on tubes. I've seen companies, like Audible Illusions, recommend NOT to use NOS tubes because their design is tough for older tubes. They don't like folks spending hundreds of dollars and having tubes die within a couple of months. Many new tubes may sound a tad grainier, but are built more ruggedly. So it's possible that the tube designer is just trying to warn folks the their designs may be more treacherous on tubes.
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