Pretty cool that you have and still use a Dual 75. I have a Dual 75a that i bought from a dealer in around 1974 and have maintained it over the years, had it recapped, etc. several years ago, and just ordered a new set of output tubes from ARC. I don't really use the amp anymore, but have kept it for sentimental value. Out of curiosity, what did you use to plug into those quarter inch jacks to bias the tubes? I have a meter, so that's no big deal, but I'm not sure how those jacks are wired.
And, fwiw, different tube equipment would seem to react differently to burn in, in terms of sound. I just retubed my SET Lamm amps, I really could detect no shortcoming in hearing the new tubes, in fact, the amp probably sounded better. In burning in my new phono stage recently, (an Allnic) it took forever, and the sound changed dramatically over time.
And, fwiw, different tube equipment would seem to react differently to burn in, in terms of sound. I just retubed my SET Lamm amps, I really could detect no shortcoming in hearing the new tubes, in fact, the amp probably sounded better. In burning in my new phono stage recently, (an Allnic) it took forever, and the sound changed dramatically over time.