From your profile, it appears you have a decent source and choice of single ended, solid state amps. I'd look at a Bottlehead Crack with speedball to add the choice of tube goodness.
You really don't need to spend megabucks to hear the HD650 at its best. This notion that it transforms into an uber headphone is a remnant from when the HD650 was at the top and the market offered less choice in quality, affordable components (amps, dacs).
I understand the M-Dac has xlr outputs, so you could try going the balanced route to hear what the fuss is about. The Mjolnir is a fine amp for the HD650 in balanced mode. However, I'd only recommend that if you had other headphones for it (orthos perhaps down the track). Otherwise it's overkill, IMO. The HD650 ceiling isn't as vaunted as what some posts would have you believe.
Thank you, olor1n (Headphoneus Supremus, all hail!) I like the look of the Mjolnir. I am eventually thinking of the HD800s, but I will need to demo them somewhere. Thank you for your kind advice. First the source upgrade, then onwards and upwards, feeling and hearing my way.