Had wanted the HD600 for something like 15 years, but could never justify the price. Life moved on, and the 6XX became a thing. In those 15 years, I only owned Sennheisers, never gave me a reason to leave. Started with a pair of HD200s which lasted probably 12 years until the leatherette flaked off. Replaced them with HD598SE, huge improvement!
But the 6XX. Dang. Yo. I got them two days ago, and immediately tried back-to-back-ing some tracks I usually listen to. Was absolutely astonished - I'd always considered myself reasonably picky about headphone quality (and the 598 is pretty well respected) - but the 6XX is a whole different class.
My test was using tracks like '347 Midnight Demons - Carpenter Brut', 'Beam - Volkor X', '6.24 - Danger', and so on. Directly plugged into my 13" Macbook Pro. No EQ tools or anything, just the straight output from Spotify. Starting with the 598s, I turned the volume up as loud as I could handle, listened for 10-15s, paused, unplugged and plugged in the 6XX, listened for 10-15s, switched back. Did that 2-3 times per track. The 6XX requires almost full volume, where the 598 I could only take 85% volume - makes sense, 6XX are harder to drive.
The 598 has more bass, but less precise. The treble/mids were almost physically painful after switching back and forth.
The 6XX has more low-end (deliciously so - think the difference between a 12" and 15" sub in a car), but less bass, if that makes sense, but EVERYTHING ELSE sounds like the same jump I experienced when going from a $10 pair of headphones to a $100 pair. Listening to music has become a complete joy again. Not sure I'll spring for an amp, as I travel a lot, and so don't want to add more 'stuff' to carry around (i.e. I prefer a more consistent experience) - and another $300 seems a bit much.
The only downside to the 6XX is the frame dimensions are narrower, and I have a giant head. Thankfully the straps are metal-banded, so I just bent them until it was comfortable. I'd even go so far as to say the 598s are more comfortable, a little lighter - if I could get 6XX sound in a 598 frame...perfect. The 598s are on the shelf as backup headphones only now.
On a less '6XX' note, DHL into Canada comes with a $44CAD C.O.D., so remember to add that to the price. With current exchange rate, shipping, C.O.D. charge, these ran me ~$315CAD. Still super duper happy to have HD600 sound for not HD600 price.