Note - This is a non-audophile('s) review.
Just started using this. I am yet to connect it to an amp. I have directly connected to a DAP, Shanling M0. Even without a dedicated DAC or amp, I feel ecstatic hearing Dire Straits (Sultans of swing in Live Aid), Cream (I feel free), Cat Stevens, Pink Floyd and Eminem. Its 2 AM, and I don't feel like sleeping yet.
An advantage for us less-knowledgeable folks (about audio gear, terms, etc) is that we don't know what to expect. I don't understand sibilance, warm, thin, wide sound stage, mids, highs, frequency response, etc. I just know when my soul is fed with gourmet food, and when it's fed with junk. I am feasting now, as I salute Mark Knopfler and the creators of this headphone.
Update 1 - With Centrance HD DAC port as DAC and laptop as music source - In high gain, the effect is as good as an amplifier, because it's very loud and very clear.
Update 2 - Used 6xx with Obj 2 amp + SMSL Sanskrit 10 MKII DAC. Also used it with E1DA 9038S dac+amp. Absolutely phenomenal sound on both combinations. 6xx is a winner all the way.
And where is the tight clamp force I read about? This is super-easy to wear, even for hours. Working from home for months now, and 6xx is alsmost an extension of me. An hour a day with some great music through it, keeps the blues away.
Would recommend to a friend.