I had to move my studio setup from a room with great acoustic treatment (we had a fire), to a room that’s impossible to treat, and on that basis I was forced to have more trust in my cans, than I have in my SE Munroe Egg 150 monitors.
If I can listen to music I know and love via that correction profile, then I can produce music with that profile too. At 100% things can sound a tad too boxy in the lows, for me it turned out to be a very ‘unscientific’ 63% mix that matched closest to my speakers. The trick with an app like Toneboosters Morphit is the wet dry mix. There’s little need for a headphone amplifier if you’re using a decent audio interface. You’ll notice subtle (or not so subtle) differences and that will equip you with the knowledge that you need to trust your cans. Outside of paying someone like Sonarworks to create a custom correction profile for you with proper measuring equipment, the way you’ll come to trust a pair of high end cans for music production, is to spend some time with a collection of your favourite music and then listen to it repeatedly via both speakers and headphone (only if you have monitor speakers you trust). Having said that I definitely think that the 660s is an improvement on the older model when used with a decent correction profile. The core difference between the older HD 650 model and the updated 660s model is that the bottom end was more natural in the 650’s without relying on a correction profile. And that was only after using a custom correction profile in Toneboosters on iOS (I still use Sonarworks on the desktop). It took about 6 weeks before I could trust my HD 660s’s. I used to laugh at those on audiophile websites with their discussions regarding lengthy burn-in and sexy headphone amplifiers, but burn-in can make a huge difference if you want and need to trust your cans as an alternative to a decent monitor/amplifier combo in your studio. The essential trick with any high end set of cans is allowing for burn-in. And when these go, I’ll be searching for another open-back pair I searched in vain for another second-hand pair but eventually purchased a pair of HD 660s’s. I used Sennheizer HD 650’s for years until I accidentally ruined one of the cans.