Pros: Great isolation, reasonably comfortable even with glasses. Very listenable sound signature.
Cons: Require a good amp, not the headphones for use with your iphone or tablet. pads are fragile, wire between cups is flimsy and subject to breaking.
The sound signature on these can be described as almost "Grado"esque. They have a mid/high forward sound that makes them great for guitar rock. They don't have the harsh treble that is characteristic of most of the Grado line and a lot of the other Superlux models. Soundstage is good for a closed can and better than the HM5. Compared to the Brainwavz HM5, I find the 660 to be tighter and more controlled bass with slightly more exaggerated mids. Compared to the ISK-2011 I find the sound more V shaped and more fun. The ISK is a good monitoring headphone but is not nearly as musical for the genres I typically listen to.
For Rock music, I would rate them in this order:
It isn't really fair to include the 325 since the price is 5x that of the others in this list but the sound signature draws comparisons so I included it. All of the rest of these phones fall into the sub-$50 range and draw comparison based on what is the best spend for $50.
I did find that the HD-660s say they are 150ohm drivers and they do require an amp that can really push a 300ohm phone to do their best work. These are not the headphones for your iphone or Samsung. Running a Fiio E07k or a Ibasso Dzero are both better but to really push them, the Asgard and the Fiio E09K docked with an E17 are better yet. With the slightly bright sound to the HD-660, they make a good pairing with the Asgard.
Overall, these rank high on my list for most enjoyable. I find myself wearing them more often than most of my open headphones because it allows me to listen in comfort while others watch TV etc.