Generally I preferred internal amp if it is good in first place ( more transparency to the DAC ) I tried and a better measure amp - schiit heresy - with M500 dac and I lost some detailed compared to directly from M500
One of my friend got A90 and dislike it so bad. he said he only can use high gain @ 12 to hear very well. He used chord qutest at 3v output.
His headphone AKG712 and LCDX, Any thought what goes wrong for him ?