The amps you mentioned are not fit to drive the 6XX. They crave tons of voltage and the Senns can swing upwards of 500 ohms. The units you listed are suggested to 150 ohms max.
You'd be better off looking at the JDS lads Atom, Schiit Magni, etc. as they will have more than enough power for your needs and leave with plenty of headroom. Unless your source device is a jittery-noisy mess, you'll be fine not getting a DAC. But, if you really want to get one, the Grace SDAC here is solid and only $80.
My suggestions should give you ample clean power and push just about anything that isn't a current hungry planar. I really can't think of a better jumping off point.