Apart from GSD and superpowers, there's another system, called PAUL [1]. It apparently requires fewer tokens compared to GSD, as it does not use subagents, but keeps all in one session.
A detailed comparison with GSD is part of the repo [2].
There is actually an interesting mathematical method for piano tuning that takes harmonics into account [1]. The core idea is to minimize the integral over the logarithm of the sum of all spectra. This basically favors spectra that are smeared out less. Instead of the logarithm, one could also use another sublinear function, such as the square root, but I guess it just makes for a better story to call it „piano entropy tuner“.
The paper also shows a nice plot about the „stretched tuning“ that you mentioned.
[1] https://github.com/ChristopherKahler/paul
[2] https://github.com/ChristopherKahler/paul/blob/main/PAUL-VS-...