I'm sorry to see that you haven't had a reply. I'll try.
I've had the Kann Cube for a few weeks. In general, I think CIEM and IEM headphones can sound good. I think your HD650 can do good. I briefly owned the WH-1000XM3 and I am not a fan.
My Fostex TH-909 never sounded better than with the Kann Cube. Since they are low impedance, I would guess that they pair better than any of the Sennheiser headphones. Sennheiser and A&K fans may disagree. After-all, the Kann Cube is built for power and Sennheiser HD800S is the legendary reference headphone everyone wants to own. My opinion is that I have owned the HD800 (previous to the HD800S) and always felt that it needed a massive amp to unleash its potential.
I also have a MrSpeakers EtherFlow C. It is my favorite closed headphone but I don't think it scales as well with the Kann Cube. I suspect that other planar headphones will need more amplification to reach their potential. Since writing this, I've found that putting the amp in "High Gain" helps all my headphones including the EtherFlow C. It's not about the volume, it's about passing more current through the cans.
Lastly, my JH Audio Layla scales extremely well with the Kann Cube. I auditioned the SP2000 Stainless and Copper (the Copper sounds better to my ears) but the Kann Cube is in a category of its own. I purchased the Kann Cube because of how it pairs with the JH Audio Layla.
Coming full circle: if budget is a concern, a good quality balanced cable for your HD650 should prove very revealing.