In case anyone wants to experience Just Intonation major and minor thirds, you can do it easily on any standard guitar or bass. First learn where the 4th, 5th and 6th harmonic points are on a string and practice touch-and-release at these points to get clear, long-ringing harmonics. Use a clean and bright guitar tone and use a pick with above average force. You can websearch for more detailed help with the technique needed to do this. Warning: there are a lot of misleading, confusing, overcomplex and inaccurate diagrams on the internet for these positions, i'm looking for a good one. But roughly: 4th harmonic: fret 5 5th harmonic: fret 3.9 6th harmonic fret 3.2 (7th harmonic: fret 2.7) JI major third: > Tune a pair of adjacent open strings to a standard major third interval, best to tune up the lower by a semitone. > With your fretting hand, hold one finger at the 5th harmonic point and another finger at the 4th harmonic point and hold this position while you touch-and-release, this will help you play the harmonics quickly in succession. > Play the 5th harmonic of the lower string then immediately after play the 4th harmonic of the higher, let these ring together and listen, you will hear 2 almost identical pitches. > Do the same again several times while slightly detuning the higher string to get these 2 harmonics precisely in tune with each other. > Then play both open strings to play the JI major third. JI minor third: > Tune a pair of adjacent open strings to a standard minor third interval, best to tune up the lower by a semitone and detune the higher by a semitone. > Then continue as before but tune the 5th harmonic of the higher string to the 6th harmonic of the lower by slightly uptuning the higher string. Being used to 12ET you may find these sound slightly 'out of tune' due to conditioning but also certainly 'more in tune'.