Where to start upgrading?

I have had a headphone system for decades. Over the last ten or fifteen years I finally buckled down to really make it great. It took many years. I went as far as importing a headphone amp as well as buying several others. For headphones the most important component is the headphone amp... period... OK, within reason not a $25 steamer with it. 

The company that changed my perception of what was possible was Woo. They specialize in headphone tube amps. I purchased a WA6 (fantastic but limited power), then the two component WA6SE (fantastic but a bit more power... but limited) and the lots of non-Woo solid state headphone amps. Finally I bought the Woo WA5LE. That amp (see my virtual system) completely changed everything. It is so powerful as to be able to power speakers, it is a 300B tube amp, so is incredibly warm and natural. The combination takes any headphone (especially Sennheiser) and grabs it and tell it who is boss.The power, natural musical sound that comes out is incredible. I have Sennheiser 800, Focal Utopia... etc. In each case they have never remotely sounded as good... not even close. The amazing thing, is when headphones have this kind of midrange bloom and power... they are satisfying from a power perspective... like a big main system is. When I listen to them, I don’t crave going to my main system to get the satisfaction of the bass hitting my chest. 

If you really want to make a leap that will completely change your system. Get a Woo WA5... later upgrade the 300B tubes to Takatsuki. Then upgrade your inputs... each upgrade will be amazing... with WA5 in place. 

I seldom say, "get this". But the way you phrased your question, it seems like the right thing. 

Thank you!  That's exactly the advice I was looking for.  After reading back my initial post, I think I could simplify the question by saying, "if I spend $5,000 on a turntable, will my other components hold me back from noticing a difference".  It makes sense to upgrade the headphone amp to get the most out of the headphones and move on from that point.  

that upgrade path can be sometimes tough to figure out. Used to have nearly the same HP system as the OP. You can start with the table and cart or the HP amp. You should be able to hear the improvements as make the upgrades. think there are two paths you can go. Start with a better HP amp like the quicksilver HP amp. Then go to a technics 1300 or 1200G with the cart of your choice. Instead of using the Denon cart with the Zen phono buy a decent SUT and run the Denon into the MM stage of the Zen. 

There are a lot of wonderful tube headphone amps out there and I've had several - Woo certainly makes some great ones in varying price ranges, and I've had that Quicksilver HP amp that @johnss mentioned; great bang for the buck, that is! But there are also companies like Ampsandsound, Decware, Donald North Audio in the US, and Auris Audio and Feliks in Eastern Europe who make great gear worth investigating (though Decware and DNA have long waiting lists). WE 300B tubes are also excellent.... I even drive my Susvaras and Modhouse Tungstens through some of these relatively low wattage amps and they sound just fine. 

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

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

@williewonka Thanks for the information! I've honestly not put much thought into cables, but again this makes a lot of sense. I'm excited to dive into the research a little more.

This has been great! I really felt like I was floundering with the process. Although there's way more to analyze, I feel like I've been able to direct my focus through these responses. Thanks again to all!

Better to think about what makes a “good” cable and why in terms of the physics. That is a thorny set of questions because it depends upon the nature of the output of the driver and the input of the driven component. In my opinion it’s a bit less critical for interconnects, except for phono and SUT, than it is for speakers.