I too own both the HTF600 and HM3. My HTF600 has a lot more burn-in time that my HM3, but the HM3 still sounds much better.
Quick thoughts on the HM3. Even with little burn-in they have a great sound to them and I look forward to what they will become.
Comfort wise, you really need to bend them a lot to shape them to your head. Not too hard to do though. I have smaller ears, so they are Over-the-Ear for me, but just barely. This causes my ears to get sore as they are slightly smooshed in between the pads. The pads also do make my ears a little warm, but not horribly so. I do wish the pads where oval instead of round, I would like to find velor replacements.
If I could morph the sound of the HM3 with the comfort of the HTF600, they would be the perfect pair of inexpessive headphones.
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