Some people claim there are synergies between certain tube combinations. There may well be, but the added benefit is small and trying to find reliable information on tubes, let alone two specific tubes in combination, is almost impossible. The bottom line is that almost every tube I've tried has sounded good in my Crack.
I'll let other comment on the DAC, as I'm not a big believer in differences between DACs once you get above the $100 price point and provided you account for differences in output signal strength.
As for the HD650s....just....don't. The HD600s are better. I've owned three pairs of HD650s. Each time I keep telling myself they can't be as bad as I remember, and each time I confirm that, yes, they are as bad as I remember. I know the HD650s are newer, have a higher number and cost more, but they are still worse than the HD580s/HD600s. Save your cash for something else. If you want an upgrade from the HD600s, save your pennies and buy a pair of HD800s when you can. They are absolutely amazing, especially out of a Crack. Skip the HD650s and HD700s.