I think class A is often associated with a warmer character but not all Class A amps share this sonic character, just like not all Tube amps are warm sounding per se. Jot 2, for example, is class A but sounds closer to the neutral-sounding THX 789 which is class D, but A90 sounds much warmer than 789 which is also class D.
I actually have both amps in question and an Arya V2. It can be kinda difficult to hear the stage differences between amps with Arya since it's already a wide staging headphone. The differences in stage are a little easier to hear through a HE6SE.
A song I had picked apart and would like to share as an example is "Flowers" by GeeJay. I volume matched 789 and Soloist to the vocalist's sustained note from 2:28-2:32 as close as possible. From the chorus (0:59 to 1:19), you can hear the bass on 3XP has more weight and texture. You can also hear the decay of the piano a little better through 3XP, whereas the 789 "hides" the decay a little more and thus gets lost in the background. Second verse (1:19 to 1:39) there's a little synth you can hear riffing above (and possibly slightly to the right of) the vocalists position. The leading edge of each note is blunted in the 789, smearing the movement between notes. On the Soloist, the leading edges are better defined and effectively makes it sound like it isn't buried in the background. Second pre-chorus (1:38 to 1:48) there's a certain timbre to the bass synth stabs that sound different between each note, whereas each note sounds texturally similar as the next on the 789. Finally, at the piano solo at the end of the song (2:08-2:49), you can appreciate the piano's decay better on Soloist and again has better reproduction of the leading edge here; most evidently with the piano's lower notes being played with the pianists left hand. Towards the end of the solo as the pianist hits higher notes (2:37-2:48), the 789 sounds thinner and dynamically cannot keep up with the song, whereas the Soloist conveys the piano's crescendo more accurately with weight and cleanliness of each note.
I have to say the differences between amps can be subtle a lot of the time but whether that difference justifies the price difference is up to each listener individually. I hope this comparison has been helpful.
I like how I'm not the only one with a Qutest / Soloist combo and am upgrading from a 789.
The only downside of the Soloist to my ears is the emphasized treble. I usually hear it in "trap hi-hats" and sometimes in snare drums. Does the Super Charger alleviate the "emphasis" to these areas?