My advice would be to get the Mimby instead of the MB card. It's a bit more, but worth it in my experience. Also, synergy between headphone and amp is more important than specs. I really like my Jot, but the Lyr3 / MCTH would be excellent alternatives to look into for your HD6XX. Possibly a pre-owned ZDS Jr., if you can find one, although the one I heard I did not like. The Foster headphone isn't that picky and doesn't scale up as well. I think the MCTH would be a better match than the Jot, depending on the tube used. It's certainly a V shaped headphone, and I think the MCTH would match well. (You'll also want aftermarket pads, as the stock pads are uncomfortable.) I haven't heard the HFM you purchased, so I'll offer no advice there. I'm not trying to talk you out of the Jot. As I said, I really like mine, but I think the affordable tube hybrids offer pros (and cons) that make them worth while to consider.