Thank you for the review which I thought very fair - it’s also nice to hear what a well set-up version can sound like in capable hands too. It was very interesting too to hear about sound spectrum. I like my Ukes to have a good balanced Bass response rather than to be overly biased towards the higher notes. To me Sopranos should be broad ability musical instruments - ideally melodic - rather than (for want of better expression) primarily shrill percussive sound boxes that cut through other sounds, but ideas of what a Soprano should sound like vary.
I have both a Kala KA-P and KA-S, both have been with me for some years and they are (IMHO) close in build to the KA-15S. As far as I can see the KA15-S is a stripped back version of the KA-S which allows Kala to compete at two price points, I’m generally ‘tight fisted’ but if someone can afford then to my mind the KA-S’s additional features are worth the additional spend. I note that the Uke that you played was well set-up by the previous owner and fitted with custom strings, similar work has made an enormous difference to my two Kala’s - buyers of new Ukes should note that their Ukes will probably not sound as well as yours did. I have also fitted bone nuts and saddles to mine and spaced out the strings at the nut; the strings that I use (my preference) are Martin M600’s and as alternatives an acquaintance is delighted with Worth BM’s on their KA-15S and I found regular Fremont Blacklines to be worth fitting too. My KA-S now punches well above its price point - the more I play it the better it sounds too - and I suspect that a similarly re-worked KA-15S would similarly benefit. My KA-P was bought several years before my KA-S, both are a delight to play, by a quite small margin I prefer the S model (with, to my ears, its marginally greater bass response) to be the P.
Personally I’d prefer friction tuners on those Ukes of mine (it’s a personal preference, at some point I might fit some and unlike the vast majority of people I don’t struggle with friction tuners); the buttons on the supplied tuners are perhaps on the big side but for me their size is barely even a minor issue. There are numbers inside of my Kalas that whilst not serial numbers do, IIRC, relate to month and year of manufacture. Somewhere here on UU there’s a thread about dating Kalas.