Where to start upgrading?

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

i think ghdprentice is right.

The synergy headphone /amplifier was very hard to reach for my K340...

I dont doubt his choice for driving  headphone picking the last model of Woo...

It is out of my budget as are  High end known headphone...

 I am lucky because the K340 was AKG flagship and with a unique design...

 The only amp i use with it is one of the Sansui alpha one of the best amplifier ever designed by  the Sansui  company before his death ...

This is my advice too for speakers :

As far as two channel in listening goes, I’d look into acoustic treatments before upgrading components.

Thanks everyone for the guidance! It looks like I've got to do some digging. "Plan the upgrade path".  It's so obvious, but somehow i missed it. I was definitely looking at it too singularly...

@fignuts33 - Well, having spent many years investigating and building cables, the only place I would start is with the cables.

WHY cables?

• good cables convey the most delicate signals and deliver subtle venue acoustics
• mediocre/bad cables introduce interference, which you will easily hear using headphones
• good cables ensure the components are operating to optimum levels - they connect the neutral sides of attached components
• good cables deliver the best image possible
• good cables will deliver excellent dynamic performance

So - I would do the following

• invest in s single run silver harness from the cartridge to the phono stage
• invest in a good phono preamp - I prefer Simaudio Moon  phono amps - great bang for the buck
• invest in great cables from the phono stage to the preamp
• Only then would I consider upgrading other equipment

Good cables will allow you to really hear the improvements as you upgrade other components.

What are good cables?

• Take a read of this thread Audiogon Discussion Forum

• The cable companies I typically recommend include

  • Zavfino - there silver cables are great bang for the buck - very dynamic
  • In-Akustik - very quiet
  • Hijiri - excellent cables, but expensive

Regards - Steve