As much as I’d like these to be an upgrade it’s possibly they aren’t one when both are driven properly because honestly the HD 700(when corrected) isn’t really much of an upgrade over the HD 600/650 in terms of overall driver fidelity. It’s possible the drivers are essentially retuned HD 700 but I could be wrong. You don’t get a truly notable jump in driver fidelity from Sennheiser until you get the HD 800. It’s why I’m holding back and waiting on numerous reviews to see if it’s actually a better headphone or not. If it ends up being like how you say it could be I may just get one of the older siblings as my system is better suited for high impedance headphones. I’m expecting it to be a more efficient version of the HD 650 that does some things better but maybe not everything. The impedance difference will definitely change amp matching which will make it a little more difficult to compare. The HD 600 vs HD 650 debate still ensues and this headphone will likely just add another headphone to debate getting alongside the HD 600/650. Actually wouldn’t mind a little more vocal range and midrange presence over the HD 650 more akin to what is found on the HD 600 as I feel the HD 650 weakness is vocals for me as it fails to grab me like the HD 600s vocals can but the HD 600 doesn’t seem to be able to create the same euphony the HD 650 can. I am really hoping the HD 660S can find that much desired middle-ground between the HD 600/650 that has been long requested.