I agree with the recommendation of upgrading the headphone amp first and going the tube route would be my personal preference as well. I really, really like the Decware CSP3 headphone amp/line stage preamp. You can find used examples, if you are patient, for under $1500. I also like the 300B units as well.
Then address the turntable/cartridge/phono preamp. And don't skimp on the phono preamp, it can be the most important part of the three.
And to not add to the upgrade paralysis (lol), a few other suggestions/things to not loose sight of...
- You didn't mention cabling, but you can cripple a system with cables that aren't up to the level of rest of the system. That includes interconnects, power, AND the headphone cables.
- Make sure the headphone amp can support the higher 300 ohm impedance of the HD 650's. Most good amps should be able to handle it.
- How noisy and stable is your AC power? Do you need a power conditioner?
- Are you using any digital sources? Streaming or CD? If so, how is that going to impact things.
- You mentioned that 90% of your listening is headphones. So, don't forget and loose sight of the other 10% that you may want/need to integrate into the system.
In the end, take your time, do your research and plan out the upgrade path.
... just my 2 cents...
- Jeff