Congrats on your HRD maxed! You won't be disappointed, and the appreciation grows the more you use it and the more you learn about and experience other amps/DACs.
It has great synergy with my 3 cans. A lot depends on source, though. I get the best SQ out of the USB. The most musical is the GS-1000 (BD), but not very far behind is the K-701 (stock cable). The HD-650 (Cardas) sounds great, too, but in comparison, I hear the dreaded veil over the bass. It's slight and not noticeable, however, when I use it exclusively.
The 701 is the most comfortable listen. Clear, accurate as hell, natural. You forget that you're listening via headphones. It doesn't add any color. Just you and the music. You can listen and think at the same time, and fatigue from listening doesn't set in. So, if you listen while working on the computer, it's ideal.
For WOW! I turn to the GSK. But I can't accomplish much work while wearing these. They grab and hold your attention.
For an in-between experience, I go with the 650. It offers both comfort and dynamics, but less than the other two.
I think you're making good decisions, and you can't go wrong with the 650 if you decide to go with it.