The Schiit combo should be pretty good for the HD650, as it's on the clearer side of amps.
And yeah, the bass isn't as emphasized, because the HD650 is pretty balanced. One of the flattest frequency responses I've seen.
Also don't just quickly compare one to another. Spend some time with the HD650, like a WEEK or so. Then go back to the Siberia. The difference will be more jarring. You're so used to the Siberias, which is why your opinion is skewed towards them.
Edited by Mad Lust Envy - 1/30/13 at 2:47pm