I am running the headphones through my Focusrite Solo. Is that the issue?

Almost certainly not.

Have you done any eq correction? U/oratory1990 tests headphones and gives EQ recipes to apply the Harman curve, which helps pretty much every headphone to sound its best (in my opinion).

Here is his recipe for the Sundaras

Replacing your DAC or amp will give basically no change in sound whatsoever. You’ll be relying on the placebo effect to convince you that you’re happier.

Always do what sounds most pleasant to you. Who cares what a chart says. The chart isn’t listening.

I do have a few songs that I can only really tolerate listening to on my car speakers, because my good headphones and my good main system show me just how badly mixed and mastered the music is.

Set your phone on the speaker stands, and drape the cables over top like a garnish. Fantastic sound.