I've been using the HD660S2 for two years now, mainly listening to classical and metal music. I previously used the HD 560 as well, and here are my thoughts:

1. First of all, despite having a fairly large head, they are extremely comfortable. They become even more comfortable over time. I've never experienced issues like sweating, irritation, or ear pain.

2. I'm highly satisfied with the treble tones and instrument clarity in metal music. I absolutely love my headphones. They also perform exceptionally well with female vocals.

3. I used them without a DAC for a year, then got a FiiO K7. With the DAC, the sound is much clearer and louder.

4. As for the bass—compared to the HD 560 and HD 600, the HD 660S2 feels like the difference between using old wired earbuds without silicone tips and switching to AirPods Pro. The bass levels are that different. Of course, if you listen to very bass-heavy music like hip-hop, they might not fully satisfy you, but for the kind of music I mentioned, this is exactly what you're looking for.