AFFORDABLE (under $1K) Best SS and Tube Amps for HD600

Best SS Under $1K for HD600 (or similar load) __________________________________________________________

WHAMMY Class-A headphone amplifier w/ OPA2107 upgrade OR 2 OPA627 pro build ONLY $500-$600 PASS Design (similar to/ better than a dynalo- warm + clinical - choice of chip switches sound - these are two I like best- choices chips R infinite)
TCA HPA-1 ( Tom Christiansen Audio ) $899 (warm and clinical but not like WHAMMY

Gilmore Lite Mk1 or Mk2 (Like a Dynalo) $300-499 (nice- a bit warm/clear for SS)
Rupert Neve Designs RNHP $499 (clinical gain power)
neurochrome HP-1 Kit $500 (similar to DYNALO and TCA HPA-1)
Monolith by Monoprice THX AAA Balanced Headphone Amplifier featuring THX AAA 887 $399 (cheap and very good black velvet background- just nice warm sounding like WHAMMY or TCA or Gilmore)
Emotiva BasX A-100 Flex with jumpers activated for full headphone power $229 (gobs of raw power no real special sound except its fun to open the HD600 with this much raw power for cheap)

Best Tube Under $1K for HD600 (or similar load)
Quicksilver headphone amp $899 (probably best looking of all these and sounding)
Bottlehead Crack w/speedball and upgraded power cord by them Professionally build ($800) (HD600+this is like peanut butter and jelly - everyone agrees this is nirvana for HD600)

TOR AUDIO Roger $560 (amazing like Crack without speedball/power cable) DIY Bottlehead Crack w/speedball $400+ 90 for cable + 8 hours time and no where same build as above
Bottlead Crackatwoa 800-$944 KIT ONLY qualifies here under $1K BUT you could get pro built for ($1700+)
Schitt Valhalla $349Schitt
Schitt Mjolnir $849
Woo WA3 Headphone Amplifier $599 (no longer made - find on used market or wait for ver2)Woo WA6 Version 2 $699
Woo Fireflies with DAC 2nd gen $999 Starving Student II Headphone Amplifier $229+ 8hrs buildTOR NICK Headphone Tube Amp $480 (wood w/tubes)
Im probably missing a bunch but this is my short list! Please join in...
Honorable mention OVER $2500 probably no one EVER heard of: Toolshed Amps The Darling HEAD AMP made in USA $2595
The Cavalli CTH tube hybrid amp from Massdrop gets very nice reviews from Sennheiser owners who say there’s great synergy.  At only $175 it sounds like quite a bargain, and I’m strongly considering it for my Hifiman HE400S. 
yes I heard the same thing- nice amp for $$$ for HD600 or H6XXthis review says the Schiit Vali 2 is better for even less
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Just discovered this little Chinese Tube gem La Figaro 339i Tube Headphone Amplifier $650-$900 -
FROM A REVIEW SOMEONE ELSE DID: "I have to say, how wrong it might sound, that I love all of my headphones with the 339 but the ones standing out most are the Sennheiser HD600/650 and the Hifiman-HE500. The Sennheiser – 339 combo is really a match made in heaven and I have never heard the Senns sound as good as with this amplifier. When I have people over, even those not knowing much about audio, they all put this combination on the first place of the podium. It’s just that good, warm, smooth and detailed. The perfect Sennheiser sound with good bass and detail.Compared to the Bottlehead Crack, the OTL Crack is the faster amp of the two, it it clearer sounding and focuses more on the treble. Bass with the Crack is punchier but not as deep and it doesn’t quite have the same rumble. Overall the 339 is smoother, warmer and laid back with an even bigger sound stage. It just makes you feel the music better (softer) as the crack. The Crack is a great deal and an excellent amp as we already said in our review but the La Figaro 339 is the better amp for me. Compared to the Woo Audio 2, the 339 is the more tube-ish sounding amplifier with a much warmer sound. The WA2 sounds much cleaner and faster but misses the warmth and soul I like of the 339. The WA2 is more analytical as the 339 and the bass depth and rumble isn’t quite the same, however it does have an even bigger sound stage. Most likely a full WA2 review will follow later. "

He actually didn’t say it was better — in the end he said it was “six of one and half dozen of another” but that the Vali 2 was $100 cheaper.  That’s no longer true as the CTH now goes for only $25 more.  It offers more power than the Vali 2, but the V2 counters with preamp outs.  Both very good amps I reckon and probably much more alike than different.  I’m now looking at the Eddie Current ZDT Jr. that also seems very interesting but not sure it’d have the juice to fully power my HE400S.  Anyway...
The Quickie hasn't been $899 since I've been looking. $998 and then I'll want better cans.
I think I've found a better way.
Faced a similar dilemma recently and eventually went with the Drop   THX AAA 789.  I bought blind, but based on numerous reviews, it seemed to offer the best performance vs cost. I paired it up with a SMSL SU-8 DAC (everything balanced) and the results were spectacular.  It drives all my headphones including the Sennheiser HD650 (sibling of your HD600s) with ease and aplomb. 

I had a freaky experience a few nights back. I was home alone and playing something with lots of acoustic guitar via my Monolith M1060s and I kept hearing these rustling noises behind me. 

Eventually I determined it was just the guitarist's trailing fingers brushing the strings but the image was so holographic that it was coming from behind me to the left.

I've been racking my mind trying to remember the track but... The music was familiar, but this was the first time I'd listened to it via this rig.  

I haven't explored tubes in my desktop config yet, so I can't offer any feedback there. 

I just had the opportunity to compare Op amps in the Pass WHAMMY which I find sounds better than the crack/ speedball - really- no kidding.
Comparing the OPA627 (you need two so about $60) vs OPA2017 ($20)
Using the OPA627 is a bit more clearer and articulate across the spectrum - open spacious, clear and deep bass - like opening a window to the OPA2017

I if had just the OPA2017 in the WHAMMY - I already determined it sounds better than the Crack w/sb and the Emotiva Bas-X 100 - and the two MUSES8920 op amps in my TEAC UD-301. BUT - The OPA627 dual chip design is by far best especially in the WHAMMY. I have used many amps before this- and for a single ended amp this beats even the Bottlehead Mainline 1.1 and the Crackatwoa as well. The closest the Bottlehead comes is the crack w/sb and power cord upgrade but still not revealing the last details that the Whammy provides my HD600. I can not comment why the power cord on crack makes such a nice difference but it does-

I have rotated my ER-4 Micro Pro P and S editions in the WHAMMY and its a perfect match as well. Unlike the Crack or the Emotiva which pale when using these critical ear phones.

I also used the Sony MDR-V6 and MDR-2506 which usually sound terrible when things aren’t right- they are not the best for music long term though on any system.
I also used the Master Dynamic MW30 and they sound excellent with WHAMMY as well.

very impressed.
Caveat to WHAMMY- no output RCA and no front power on off switch, and you need to rely on a professional builder- I got mine from James the HeadAmpBuilder

I can not believe how good this PASS design is- I cant stop listening to music!
Just a thought, you could get a new pair of headphones for less than $1k which may make a bigger improvement. I would probably look at Focal Elex or some Beyerdynamic phones and then save for some better amp.
Nobsound NS-08E
fitted with Nos RTC 5654 RT tube sound fantastic with HD 800s.

It cost me around 100 bucks including Nos tube and adapter.

This combination of Nob and 800s sounds more natural and pleasing than Raal Sr1a driven by Jot R while the latter sounds more open and exciting but close to hot.

This one also may work well HD600.
Im in heaven with the WHAMMY- As I sit here late at night- Im now using my Etymotic Research EM4pro S(ER4S) - to check for any noise or any imperfections in my set up- I love it- very very detailed. Bass is scary when the song has it. I just ordered the 250ohm Beyerdynamic DT-150 tanks just to see how far I can take my set up. I must be into some s&m or maybe I just want to hear every detail possible. I do love using the HD600 but the wife keeps telling me they are too loud!
In rotation as I write this....Shirley Horn You’re My Thrill, Shirley Horn Since I fell for you, Losing Hand by Harry Belafonte - you hear it all! And I wouldn’t want it any other way! enjoy the music as they say. OH my! God Bless the Child Gregory Porter from Be Good album- so gooooood.... now Linda Ronstadt Am I Blue w/ Nelson Riddle conducting

Although I had enjoyed Raal Sr1a driven by Jot R for the last three months for open and exciting sound, sometimes it got close to hot.

I got Freya+ to tame the sound of Sr1a.

I skipped stock JJ tubes and inserted Nos Amperex 6sn7 gtb in gain stage and Sylvania gtb on buffer stage into Freya.

Sr1a driven by Freya and Jot R sounds much more musical to me than driven by Jot R.

Out of curiosity, I am driving HD 800s out of Rca output of Freya using "Hosa TPR-257" adapter bought from Amazon.

Wow, Freya+ is also a killer amp for HD 800s.

I had driven HD 800s by headphone output of Dave and HMS.

It sounds transparent but with one note bass.

But with Freya in the chain after Dave, HD 800s sounds almost perfect with nice soundstage, clean details and full and textured bass.

It may also work well Hd600.
The Quicksilver is awesome !!!!    I've had it for a couple of months now and it is addictive.  I listen to phones like never before.  Well worth the price, as it is built just like their full size amps just 2/3 the size.  I have their Mid Monos and I bought the headphone amp sight unseen for that reason alone.  ... kick ass for the money.
@oddiofyl Do you need a separate preamp for the Quicksilver? I'm seriously considering it. What DAC are you using with it?

Right now it's connected to the tape out of my c-j preamp,  I did this to allow all of my sources to appear at the QS RCA input.  I'm using a NAD M51 DAC.

I first connected it to my phone via a Audioquest Forest 1/8 Stereo to RCA and even that sounded great.    I am using Grado RS2e and Klipsch HP 3.  The other night I found a pair of AKG k240 in my closet.  They were the 600 Ohm Monitor version.  I forgot I had these.  I've had them since 1982 or so ething like that.   They were in the box and put away because I never had a headphone Jack that could power them.  I plugged them in and was surprised how good they sounded.  Drove them no problem.  Same holds true for any phone you plug in,  it makes any phone sound it's best in my opinion
I also have the Neve and it is an excellent amp but not in the same league as the QS
@oddiofyl  Appreciate the detailed response. I'm almost ready to pull the trigger. Quicksilver has priced them very reasonably.
My Quicksilver Mid Mono amps are 6  years old and have been flawless,  their products are super reliable.  The headphone amp is built exactly like their power amps, heavy gauge steel chassis, quality transformers ,  plus it weighs 16 will not be disappointed. 
Adding Monolith by Monoprice Liquid Platinum Balanced Headphone Amplifier by Alex Cavalli (right now $549 was $799)
AND subtracting the Emotiva BasX A-100 Flex - its a noisy pos
I'm with oddiofyl on the Quicksilver, it's completely addicting !

I'm using it with the and HD650, HD800s and Focal Elegia
Same here ... I ordered the Quicksilver a few weeks ago, and it's just amazing. I love the lush, holographic sound. The low end it provides took my Hifiman Arya and Focal Clear to a whole new level. 
That's awesome !!!  Enjoy .... Its such a great amp.  I am now using it with a RME ADI-2 DAC and it's an incredible combo.  The RME has a very good headphone section...or sections I should say.  If I didn't own the Quicksilver I would probably be happy with it.   The best feature of the RME is just that, it's feature set.  5 band PEQ, selectable digital filters,  bass and treble, loudness.   To name a few.  

The grip that Quicksilver has with bass is scary.  Plus it is clear as a bell at any volume.  I buy and sell a lot of gear and these two are keepers,  especially the amp as it will never be obsolete....dacs do have an inherent obsolescence. 
Not sure why BasX would be removed. At $230 I think it’s fantastic at it’s price point for HD600 / HD650. I liked it just as much as the crack with speedball. BasX with iTube combo also sounds great.