I use the 789 with both balanced and single-ended output. Either way, it’s the cleanest amp I have by a significant margin. The big difference is in suddenly being able to hear very small events, such as counting how many times a singer’s voice cracked and the exact onset of clipping.
Yes, absolutely. My opinion of the Magni 3 was formed with HD-600s. I’d used them for years with everything from phones to PCs to amp output and knew them very well. The Magni 3 caused some sort of compression/distortion/flattening of the dynamics that I’d never heard before. I too had to crank the volume for the same impact I was getting even from Bluetooth earbuds. [And only later entered my balanced phase.]
Of my headphone fleet, the Elex and HE-560s are the most revealing of sources.