I have run my hd 650 with the Musical Fidelity M1 HPA + Vdac II for a long time now but guess what ? Yesterday I tried the built in dac in the M1 HPA amp and it sounds better than it does with Vdac II. With the Vdac II the sound is almost to warm and dull, a little bit fatuige in the long run. The built in dac is colder with a soundstage thats more spacey and wide. The soundstage in the V-dac is to narrow and only fits music like jazz-funk-chamber music.
I think the a bit colder sounding built in dac fits the HD-650 perfect, giving it a little sparky treble to. Actually the soundstage now is reminding me of my DT 990 i once had, and that was the best thing with the DT 990´s.