This summer I´ve listened to my K812 modded with a 0,9 meter/36 inch UP-OCC cable paired with my back packed portable rig (FiiO X5 with Van Damme Interconnect and Lovely Black Cube amp with Burson V6 Vivid). First I have to establish those headphones are not made for portable use, as they are heavy and having a loose fit on the head - don´t think about running with them!
For the sound, when listening to the right kind of music/well recorded, it´s fantastic - more intimate and direct than HD800. The sound stage is big, but the singer is more up front/closer. I let my almost 70 year´s old father listen to them, and he stated it´s like listening to real home speakers, and I agree with him - pretty good speakers to with a full bodied sound and a warm bottom.
Despite the warm/bassy sound, they have a unbelieveble clear top/treble that I will NOT say have a metallic glare to the treble. But maybe it depend´s on the rest of the chain, from the source file, player, cable´s to the amp (and don´t forget that a chain will not be stronger than it´s weakest link!) -If you put in crap, you will get crap out!
I have only used high graded parts/component´s in my amp, used gold plated copper connector´s and soldered everything myself with RoHS unleaded solder. Also every RCA and Phono jack connector are directly soldered to the cable´s without any press connection.
Music I can recommend listen to with K812 is Alicia Keys, Jessie Ware and Adele.