I have been trying to understand why my A minor shape barre chords and specifically A minor in 1st position sounds better with the B string tuned slightly flat. In equal temperament the minor and major intervals are mostly affected by being around 15cents off. Is this perhaps what I am hearing? I thought about this a bit but then shouldn't it be the other way since the interval from A to C is a minor 3rd which means the B should be 15cents sharper not flatter to be closer to Just intonation yet it sound better with the B tune 15 cents flat. Can someone please help me make heads or tails of this please.
You're right, the pure minor triad lies in the other direction (B-string tune 15 cents higher).
However, when you tune down the B-string by 33 cents, you arrive at another quite consonant chord, with A, C and E in a frequency ratio of 6:7:9. You know you're there, when the chord is completed by playing the open D string, which would be the two octaves above the fundamental note here. I'd also say the 7:8:9-chord is more consonant than the minor triad, but keep in mind, it's a different chord and it will not fit the same context as a minor triad.