Both Magni 3+ and Heresy are pretty powerful, and rated for 410mW max into 300 ohms. The Asgard 3 is rated for 600mW into the same 300 ohms, which is only 1.6dB (not that much, since full power on any of these with Drop/Senn HD 6XX would damage your hearing).
Asgard's advantages are:
1) the nicer 27mm alps volume control for improved channel balance, especially at lower volumes;
2) the continuity output stage, which IMO, is more applicable for lower impedance loads like 16 ohm planars;
3) the ability to accept a DAC card internally.
Right now though, Modi 3+ is better than the 4490 internal card, as Modi 3+ offers Unison USB, plus additional S/PDIF inputs for about the same money (but comes in a separate enclosure that doesn't stack evenly). Modi 3+ also stacks nicely with Magni 3+ / Heresy.
If your are not in the USA, then the choice may come down to availability from your local distribution and the synergy with your headphones.