What do you feel is lacking with the HD650? Dynamics, bass, detail, soundstage? What are you looking for? Same questions we ask here about ML speakers wrt amps, speakers, cables, sound, etc..
So, you're running your HD650 right from a CDP. Like ML speakers, you need a good amp (and source, of course). My recommendation would be at looking at getting a dedicated headphone amp or dac/amp. You have in no way reached what the HD650 are capable of.
Like ML speakers, it's all about synergy. You can't just plug in a pair of HD800 into a CDP and expect stellar sound. In fact, the HD800 are pretty demanding and you'll probably need to spead at least as much to drive them properly. Without a decent supporting rig, it will sound awful! (You wouldn't drive a pair of CLX from an AVR would you
)
Check out Head-Fi.org, specifically the HD650 Appreciation Thread. It may take hours (or days) to read through and there are a lot of good recommeded amps, cables and sources. There are also a lot of comparisons to other cans which may help.
I spend a lot of time in my home office and have spent the last two years trying to nail down a decent headphone system. Spent a lot of time on HeadFi, researching and auditioning. I ended up with the Schiit Lyr amp and it's stellar with the HD650. You can also roll tubes to tailor the sound. (I've recently moved on to the HD700 and HiFiMAN HE-400 but had the HD650 for 2 years - almost regret going to the HD700 but it has worked out well for me - next move is the LCD2r2)