To test the HD 660 S I used the headphone for over three weeks, comparing it directly against the HD 650.
My test setup consists of the Schiit Audio Modi 2 DAC, as well as the Schiit Audio Magni 3. Both cost £110 from the Schiit Audio Europe website, making the combination (AKA the Schiit Stack) a cost-effective amp/DAC setup that still delivers good results.
I used the HD 660 S stock 6.35mm cable, plugging directly into the Magni 3. Given the lower impedance of 150Ω, I typically held the volume dial at around 12 o’clock. The HD 650 is more demanding, and typically I had the volume set at 3 o’clock when using that headphone.
All of my music is streamed via Tidal HiFi with maximum quality settings enabled, giving 24-bit/192kHz streams.