There have been times on the PW forum when I should have just listened rather than talk/write… this may be one of those times, but what the heck, we live and learn.
There is a difference between “tuning stability” and a piano remaining in perfect tune for extended periods of time (which is usually not the case). No piano, regardless of how fine or how skilled the tuner may be, will remain in perfect tune for an extended period of time.
I don’t have the reference in front of me, but I read about an experiment done by a very well known, highly regarded piano manufacturer, in order to test the relative tuning stability of its pianos. If my memory serves me correctly, the piano, placed in a controlled environment, was tuned to perfection and then not played, but left alone just to test the tuning stability at static conditions. Test were done periodically to measure the tuning and it was found that the piano began to drift flat within days of the perfect tuning, without any playing or pounding at all. Was the piano new? Yes, most likely.
To me, tuning stability has to do the just how far out the piano will drift over time, even in a controlled environment. Things can be done, like adding a DC, or what every, but pianos drift out of tune naturally over time.
Yes, some tuners are better than others, and a good tuning will last longer, but no piano will remain in tune perpetually.
If this is unacceptable, get a digital.
Just my .02.