I'm comparing these to my my old Sony WH-1000XM4 which I bought for the noise cancellation; They do sound clearer/better in the mids or just overall, I don't have any negative to say about the sound per se. I'd like more bass though, and personally I like closed ones so I feel more isolated from the world.
But my point here is that honestly 600 dollars for this is already going into snake oil territory for me. They don't sound that different. And the materials and construction is fucking simple, it's plastic, foam, synthetic pads/polyester(?), very basic stuff. I could see these being sold for 30 bucks from china. And this is not a dig at these headphones specifically.
I'm very specific about image quality. Audio quality I can't feel the same way about, it is somehow half snake oil and half legit quality differences. I think what it boils down to is people make too much of a fuss about tiny audio differences.
How long have you had the Sennheisers for? It takes time to get used to the new sound signature and the pick out the differences in the songs you listen to. Also maybe they don't have the right type of sound signature for your preference. And its possible you don't have an ear for picking out the differences in the music or the differences don't really matter to you.
For the record, I think I mainly fall into that last camp. For the most part, I can enjoy most songs even with different sound signatures and I don't usually listen to music critically, so small details don't matter as much to me. You might be in the same camp as me, which would mean you would probably be satisfied with a decent quality headphone in the sub $200 range. If you're looking for a wired closed back, I'd recommend something like the AKG K371.
While there is a decent amount of snake oil in this hobby/industry, I certainly wouldn't call the HD 660S2 snake oil. There's a lot of factors as to why you might not prefer a headphone even if it is expensive, diminishing returns being a big one. Either give the Sennheisers some more time to get used to them or return them for something less expensive like the AKG K371.