I tried many amps and none where enough power to drive them with good volume when I wanted. That topping tops out at 51 mW at 300 ohms while the Magni puts out 410 mW at the same ohm. Huge difference in power. In fact, the Magni puts out more power at 600 ohms reaching 215 mW.

I really like the sound overall of the HD650, just wish it had a little more low end. But for a flat sound it is very good. Does the 600 also require a strong amp? I tried many amps with the 650 and the cheapest I could find that had enough power was a Schiit stack Magni/modi

I have HD650. I never heard the 600’s. How do they compare?