I had a Ragnarok 1 and I have a Ragnarok 2 which deliver a ton of power to the HD-600, yet every single OTL amp (BHC, BCHs, DXUOO T-26, Feliks Echo 2) all sound much much better with the 600. I agree the 650 and the 6** are too warm. I get a big 5 blob soundstage with my BHCs compared to the smaller more closed in 3 blobs with every SS amp (more than the two I mentioned) in use. The purpose of the HD-600 is correct timbre. Step down a bit on the two bright spots using PEQ in the high end, and add a tad of bass from 35-80 Hz and they will play piano and dual piano timbre correct - far better than any HFM headphone w/o PEQ (which I have own/owned 14). Its not for metal, industrial, electronica with heavy bass. It for jazz, classical and other well recorded music.
Lake People and Violectric are better than most SS. Hit Head Fi for dozens of folks that agree with me on the HD-600 stock and w or w/o PEQ.