Good choices! I've only heard good things about Feliks amps, but have not listened to one myself.
If you haven't yet heard the HD6xx on a superior OTL amp, you're in for an awesome surprise. I returned to headphone audio after a couple of years away, and the one sound I knew I wanted to get back was OTL tube amp + HD650. The CSP gives you that magical pairing in spades.
To put it in perspective, out of the CSP, my HD6xx sound 85% as good as my Verite. To the point where I may sell my Verite and just be happy with the HD6xx.
Last fun fact: I spoke to Steve a couple of weeks ago since I had a question, and discovered he also listens to ZMF Eikons. FWIW

Have fun!,
Richard