The Cayin C5 is a very nice portable amp, but I think you'll find the A2 to be more transparent and allow you to hear more detail than the C5 plus it should sound more dynamic.
It's been too long since I've heard the 650's (my son now has them) on the A2, but I have heard the 600's on the Asgard 2 recently and its a great combination. A lot depends on your source, not only your DAC but source files. Of course, even low cost DAC's are getting so good nowadays that you probably have that covered. Just be sure to use good quality source material and I think you'll be very pleased with your results. Keep in mind that Schiit Audio does allow you to return the amp if you're not happy with it.
Like I said before, if you're looking for more of that "tube-like" sound (nothing wrong with that, I love tube amps) then the A2 probably won't be want you're looking for...though the A2 is not a cold sounding solid state amp at all.
The Cayin C5 is a solid state amp, and I thoroughly loved it's sound, so I believe I will enjoy what the Asgard two brings to the table.
And as for the Asgard 1, is the Asgard 2 a direct improvement in all ways? I ask because I've heard good things about the original.