Hmm. I like the FSP most overall. With maybe 25 or 50% of my music, I like the 7506 much more, but with other music, it sounds awful, due to some peaks in the mids and treble. (I haven't properly equalized any of my headphones yet.) The FSP sounded fine with everything. Definitely the best choice, unless you want to save money and are going to use proper EQ. (Which you should do either way.) The DT 880 is more like a typical "hi-fi" headphone in the sense that it is neutral-ish, but kinda bright. Its peaks are nowhere near as nasty as the 7506. However, when I listen to music with the DT 880, I find that I am bored out of my mind! (I will probably trade it for a planar magnetic headphone.) It very easily handles everything, but is too gentle-sounding and doesn't have a good sense of dynamics. The FSP had great macrodynamics and pretty good macrodetail. I feel that the 7506 excels in microdynamics and microdetail. The cool thing about the 7506 is how natural and effortless it sounds, despite the peaks. It also has the best comfort and ease of use for me. Neither the 7506 or 880 have a great sense of physicality and impact either, at least not most of the time compared to the FSP. With the FSP, I could feel drums hitting, whereas with the others, they are more in the background. Being semi-open, the 880 naturally has a more impressive soundstage and imaging; however, this can make one feel distant from the music at times. The closed headphones sound more "normal" to me, but it depends on many factors. The FSP also has much stronger bass. But the 7506 has the best sense of clarity. And bear in mind that EQ can solve many problems with any headphone. The 7506 is by far the best value in my eyes, considering that it can be had for $85 while the FSP and DT 880 are in the $300-350 range.
Unlike many headphones, the FSP does not have any nasty peaks in the frequency response. As a result, it is on the darker side of neutral, since some of the frequencies are a little too recessed. (No headphone is going to be perfectly neutral, even if you can get it much closer with EQ.) Instead, it has a nasty resonance where the sound swells up and reflects back, which hurt my ears sometimes, mostly in the mid-range. It's like the sound was trying to get out, but had nowhere to go. Most closed headphones don't have a resonance as bad as this, and in fact, it's not really noticeable with most other closed headphones. However, it may have something to do with why the FSP has such excellent physicality and impact compared to so many other headphones. Anyway, perhaps they just meant that the highs don't murder your ears? I dunno. Your description was not specific at all. But it does lack some treble extension. Things that should be more in your face are a little too subdued sometimes. Unfortunately, most people who do serious EQ recommend that you don't increase the frequencies, but instead decrease them. Not sure how you're supposed to do that when you want to increase them in the first place.
Most music studios use studio monitor speakers for any serious work. Headphones are more like a backup and second opinion that cannot be relied upon exclusively.
I'm interested in knowing that too. I'm pretty sure the speakers sound better, though. =P
