Not at the moment, I’m working on a big post though that might help with that.
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Yes I would also like to make that comparison as I do highly enjoy the 650 sound.
For my preferences, the 600 is my least favorite, and the 650 and 660s2 are my personal favorites in the 6 series lineup especially when paired with more analytical chord gear.
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I’ve had these for a little bit now and have been directly comparing to the HD 600, and yeah they’re definitely not a definitive upgrade. I tend to eq them both to a very similar target so my impressions are mostly focused on the more supjective/technical aspect.
Relative to the 600’s, the 660S2 portray a better sense of macrodynamics and the transients are faster and more clearly defined, but the 660S2 aren’t as profficient in the detail within the trailing ends of tones and the decay of stringed instruments isn’t as natural sounding IMO. Treble is ‘sweeter(?)’ sounding on the 600. I find the biggest difference to be in the bass whereby the 660S2 is better controlled and tighter. Headstage is similar enough that it shouldn’t be considered in a purchase between these headphones, perhaps the 660S2 is a hair less 3-blobby but the difference is very slight, however the image seperation is a bit better on the 660S2.
I do think the 660S2 is noticeably less grainy and coarse sounding than the 660S version 1 which is a nice improvement but if the 600 and 660S2 were at the same price I’d still pick the 600 six out of seven days of the week.
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I love the way these headphones sound with guitars. That dip in 4.7k leaves a little more room for the 7-8k grit to come through in the rock tracks I’ve been enjoying.
The low end is warmer, but still not as impactful as I might like. iFi’s xBass gives it a little more excitement, which I think makes up for that gap. Overall, Im enjoying their sound signature and really like the look. The matte black and copper accent looks suuuper clean.
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As one of the commentators said it is but yet another variation in the 600 series. Sennheiser is milking the cow, which is fine as long as they would stayed in the price bracket of the other models. Right now it is seriously overpriced. Personally, I also think they should have stayed with the standard naming convention and have called it HD665S or 670S. Now we are in Asian naming territory where the next model could be a HD660S2XA!
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I Really like these headphones more every time I listen to them. These were my eighth Sennheiser purchase and I have to admit that I had become very disillusioned with the company. But these came out and everyone says that this Senny finally has bass! And they came with the added bonus of a higher price. 
I liked them at first and more every time I brought them out. I have about ten headphones that I rotate, so I go weeks without listening to them. After a few months the only thing I would add is that they are even better with a bit of bass boost from my iFi. They are also great with my tube amps too (Valhalla 2 and Little Dot MK9).
While they are 300 Ohms, they are very easy to drive. I listen in low gain with everything except for the Valhalla 2, which I put in high gain.
Great overall presentation, wide compatibilities, very light and comfortable. A great addition to the collection.
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Have you guys noticed the youtube review that DMX did a few months ago on the latest Audeze LCD-3? The shape of the frequency response is so similar to the HD660S2. Has anyone heard them both to compare their tunings?
I am not really an Audeze guy, but that is pretty interesting.
I want to know if this is really worth buying. Because I will go back to China next year. And the 660s2 is only $370 in China. If it is really an upgraded model I think I can purchase one.
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If you’re like me and enjoy metal music, the S2 is really good for that genre. Its really quick, not fatiguing, and is tuned for a perfect fit. I would have kept it if I didnt already have an LCD-X 2021. The S2 tuning keeps the heavy or distorted guitars at a comfortable level in the mix…whereas i could never listen to the same type of music with the eargain found on hd600/650. The S2 i loved for my extreme metal music.
It is objectively the best one in the HD-6 line – lowest THD, according to Sennheiser.
I have the S2 for a few days now, and subjectively, it has been growing a lot on me. The treble is really, really good to my ears.
That’s a bargain. You won’t regret.
The only downside – for the larger heads out there – is that it is the biggest clamping force I ever encountered on this line. And I think this is because how the headphone is shipped. Image below should say it all:
Yep. That bag is really tight for this can and that takes a toll on the clamping force.
Cheers. 
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Clamping problem solution that i used… Grab the metal band right where in enters the plastic earcup assembly. Pull apart moderately wider than your head for 30 seconds, then put them on and see. Repeat until they fit to your liking.
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