If you peruse the AVA preamp product pages a little bit, you will discover that all AVA preamps- be they tube, SS, or hybrid- already have headphone amps built in. I can vouch that they have REAL NICE headphone amps. Don't be fooled by some "boutique" headphone amps being as (or more) expensive than some AVA preamps.
This pre/dac question has come up before, and IIRC Frank didn't feel that the unit was sufficiently quiet for his liking. It is a great concept, fewer boxes, fewer wires, but concept and execution don't always match up.