I just want to say that I've gotten a lot from these 'tuning' threads the last few months. I don't have any "absolute answer", I don't think there is one. But I've experimented around a lot, and in the process, I think my ears have gotten attuned to some things I didn't hear before. I also think I understand what a compensator can do much better now, even though my steels don't have any, nor will they likely any time soon. Maybe I should rethink that position.
Like David D., I am a Ph.D. scientist (and in my case, engineer), and just love to analyze everything to death. Once I start playing, I don't analyze a bloody thing. But I need to do a lot of careful preparation before I can do that and enjoy it.
I start out with a 'straight-up' 440-ET tuning to get things close, but then tweak to my ear (and to the band, if that's where I'm at). I deliberately make compromises between the JI triads and the inversions/7th chords/diminished chords and what not, until overall, it sounds good everywhere. I'm probably not far away from ET, but there's just a bit of 'sweetening' of triads. I guess people call this 'meantone' or 'tampered' now. Oh, boy - some new buzzwords.
Without compensators, I don't see how to get good sounding inversions and more complex chords with
pure JI open triads (the open bar being the I chord). Of course, the VI A+F inversion is the real clinker, but there are others, like the V B+Eb dom7 or V A+B+Eb dom9 or II A+B+Eb+X dim7. These inevitably conflict with the I maj7 (Eb lever - or equivalently the inversion of the III minor). My X-lever also fails to work the same with different open and pedal/lever combinations.
How much of all this is cabinet drop vs. theoretical pitch differences (see David D.'s chart) is strictly empirical. Either way, it seems to imply either a very complex compensation system or some kind of compromise.
All of this is, of course, even more of a problem on the universal, since there are a lot more multi-use changes. I see why many avoid the universal approach, although I still like it.
So I compromise, which BTW, I also sometimes do on 6-string guitar. Not always, but if I know I'm going to be leaning hard on a lot of major open chords, I may tweak ET just a bit. Most experienced guitar players I know will also bend notes within a sustained chord when possible to get things to sound "right".
As I noted in some earlier posts, the trigonometric sum formula explains why this is all so much more important in slow, sustained passages. It seems to me that somehow tweaking out the beats in sustained chords is really critical, however it's done.
Another point. A lot has been made of the idea of tweaking the bar slightly when using open-bar JI tuning to tweak the the various pedal and lever combinations so that they're OK. That makes sense, but why is that any different than tuning ET, and making even smaller bar tweaks to make the open bar triads sweet? It seems to me that these are
reciprocal approaches.
Of course, if the open-bar and standard triad pedal/lever changes are the main course, then it makes sense to tune them more JI. But if that's not the case, it seems that a more ET approach may work better. I probably veer closer to ET on universal than on an S-10. In other words, choice of tuning approach seems to be more from convenience than necessity.
All this said, I agree with those who argue that "if it sounds good, it is good". This is not to say it doesn't matter how one tunes. For me, it's important to understand the
ramifications of how I tune.
Final thought: Keep 'em comin'. This really IS useful, IMO.