I have always been somewhat dubious about “subjectivity”tone timbre projection etc
Unless we are talking like for like ,and one factors, in intention of build.
As the general principal.one builds for longevity,but as lightly as possible for vibration resonance,and projection,and using materials that will facilitate that
Not normally a discussion I'd enter into but for what its worth to answer you question
"What determines a better sounding box?" for my money I'd say:
1. Above all the player.
2. Next the acoustic the instrument is played in, especially important for wind instruments.
3. And least importantly of all the instrument itself. I can't comment on historic instruments but based on what you can buy today new (and excluding small setups (less than 30 workers) which may or may not be in business in the next generation):
(a) The melodion makers have the best sound: Castagnari, Dino Baffetti (inc. Saltarelle branding they make), Serenellini
(b) If you want a more comprehensive accordion than the melodion makers (i.e. converter, freebass etc.) I'd go for the house of Bugari (i.e. Bugari, Zero Sette, Petrosa, Giulietti). If you pay more money for one of these labels as a new instrument over another (made in the same factory by the same people with the same bits) then I will start canning 'Yorkshire Fresh Air' in North, West and South flavours and place you on an expensive subscription.