I absolutely love these headphones. These cans punch well into the $500-$600 price range. Here is my current setup: Schiit Modi 3 -> JDS Labs Atom -> 6XX. I highly recommend this setup for value, budget entry. These headphones need an amplifier. I used to have the Magni 3, but replaced it with the Atom after comparing side by side and really enjoying the cleaner sound.
Here is what I've listen to: Focal Elex, Fidelio X1, HD58X, DT1990, HD800, HD800S.
My taste preference: From listening to the above, I have learned my preference is for a very warm, rich sound. In fact, I could not tolerate the treble from the Elex, DT1990, and HD800.
I love my 6XX, so much that I recently purchased a second pair. I put these cans right on the same tier as the Elex, DT1990, and possibly HD800. These cans target those looking for a very rich, intimate sound. Imaging is accurate, and soundstage is not particularly good. However, this is the trade off you make for the intimate experience. I highly recommend these headphones to anyone looking for non-fatiguing, comfortable, accurate sound that will give you years and years of enjoyment. It is hard to think of a reason to not purchase these headphones, unless you are unable to adequately amp them.