Not sure why your particular vintage amp works better. I also don't have the Asgard, but it seems to have quite a lot of power (380 mW per channel), so that's not necessarily the problem. I like vintage amps with some cans, too, but with my vintage Onkyo the HD650 sounds too bloated in the mids for my tastes. It does have more sub-bass than my Lehmann BCL, which was designed with the HD650 as a test case. Some integrated vintage amps are very musical, because they are so simple, much like the classic goal of an amp as a 'wire with gain'. Often the simplest chain is the best.
Out of curiosity, are you using the Asgard 2 in high gain? If not, do try that that. Schiit says that there's not overall feedback in high gain mode only, which may mean it's more transparent in high.
Also go back and forth with the same source in both, because you may find that the Asgard is cleaner and smoother with more detail in the highs. Or you may find the opposite, but it's hard to direct A/B test integrated amps to desktop amps sometimes because of the awkwardness of the size of vintage amps.
P.S. the Asgard uses 30 Watts, so it's not exactly a usb powered amp.
P.P.S. I'm not a schiitboy, but I am basically happy with the Magni as my first and probably last purchase from them.