The soloist is still my favorite amp for it. I don’t own an A90. But I tested the 789 for a bit and found it did an okay job—assuming you don’t have an issue with the 789’s treble. The soloist gives better bass performance and has a warmer, thicker sound. Which is also the case when paired with most headphones.
Here’s my post with impressions on the HE-6se
🔶 Hifiman HE6SE - #416 by marcgii