[billqs]

There's no denying the Sennheiser 600 series phones are solid contenders and all around excellent phones. I've owned a 650 and 6XX and they are really good buys for the price. I regularly suggest them for audiophiles getting into the hobby. The 650 was my first audiophile phone. (I had used Sennheiser since I worked in broadcasting, but we used much less expensive phones and Sennheiser mics.) There's every reason to be proud of your 660, 650, 600 even the 6XX. They truly are excellent.

However, saying that doesn't discount the fact that the HD800 and 800S also have a special place and perform extremely well at certain tasks, specifically giving you the cleanest version of exactly what you are listening to.

 

[Piccolo]

Just my own two pennoth, for what its worth, I would say that the HD600 will scale extremely well with better source and amplification. I was surprised when moving from a 'treated' Musical Fidelity xcans v2 to a Feliks Euforia AE. I was like you, a bit uncertain about keeping the HD600s so I dropped Mr Feliks an email to ask and his reply was "well we use them here in the factory during the design process". The only phones that I think would be a worthwhile upgrade would be something along the lines Hifiman Susvara or Focal Utopia and by that I mean that although you would of course get improvements with others, would those improvements be worth a thousand pounds or so?
I did have a bit of a disappointment yesterday when changing the pads for some Dekoni Elites - they made the sound worse, so it is back to the 1997 vintage ones, once they have dried out after being put through their periodic washing machine cycle - one per sock.