Asgard 3 can drive the DT990 1.8dB louder. That's all.
On Heresy you can reach 121 dB SPL peak and on the Asgard 3 122.8dB SPL.
Asgard 3 is substantially bigger in size, has a better volume control, has an onboard power supply, weighs more and can include a DAC board (which is nothing special) and has a bit more, but inaudible levels, of distortion.
When you would have both amps level matched you will not be able to tell any difference.
When you have them side by side and not level matched chances are you will have a preference.