My ears is not that sensitive yet, still learning to differentiate audio spectrum the focus is more on to achieve how to get a certain sounds. I also use mindstorm's chart for tweaking, I do not think it is accurate but it gave the idea on the direction to use. I have some notes as well on what kind of sounds that I have achieved. There are still some unknown territories like neutral and warm, which I seldom explore, mainly because I like sparkling trebles.
If you need new region's explanation, in a nutshell
J2 has a bit of increased upper mid and a bit of extended treble compared to J, but the way upper mid behaves is like taming a bigger upper mid. It does not sound warmer or leaner but because upper mid gets a bit bumped, there is a feel that it is less warm than J but the sound is still full and still warm. It is a good choice for anyone that feels J is too relaxed
J3 has extended treble, higher upper mid than J2, it is like untamed J2. It has deeper bass. But it does not sound like ESS. Applying this will put more weight on the notes, there is a feeling of majestic due to extended treble and deeper stronger bass
J2 vs U is like AKM vs ESS
J2 vs J3 is like AKM 4493 vs AKM 4497
J is like common R2R DAP (any R2R DAP not made by Hiby, sounds similar to this, tamed upper mid, tamed treble, warmth coming from a bump on low mid to bass, but not reaching sub-bass)
I will create a guide for MrWalkman's ROM later when the time is right.
@MrWalkman what is the reference for Warm and Neutral signature ? Is the warm taken from existing A series ROM ? or is it coming from another rom builder's firmware ?