Watching responses as I also have the HD6xx/650. These are my second pair of Senns and I can’t recommend them highly enough. The Massdrop mods are perfect. The deep blue looks effectively black in real life. I got a modded OEM cord that was shorter (1m) and balanced. Use them with an ES100 and it’s a perfect match for that amazing little Bluetooth DAC. I’m now looking at a wired DAP so I can listen to Hi Res. Would love the SE100 but my wallet is preventing that so have been looking closely at the SR15.