I bought these to replace my 16 year old Sennheiser 45?.
They're excellent headphones - very good sound already especially running off a proper amplifier. Very comfortable although that they are circumaural caught me by surprise at first. They have a soft cloth-like pad which fits around the ear holding the headphone itself away unlike the old ones which sat on top of the ear. It's actually quite nice as the ears feel they are more in the open.
These are fully open backed. Also the driver is further away from the ear. This means that if you want to run them at higher levels you'll get notable leakage for your neighbours to hear. I like to hear clearly what's going on around me so the complete lack of isolation is a good thing. I can overlay my music at a nice quiet level on top of the sounds of the office and still hear someone approaching or someone talking to me. In the office environment the leakage at that level isn't noticeable above the sound of computers. Ours is a fairly quiet office.
At home I can up the level a bit and get great sound, but in a quiet home anyone in the same room will hear them. Still, you can get high performance levels while those watching telly in the next room don't hear. These are very much headphones for those that want excellent sound quality and imaging from what is a very effective medium to achieve it, but not for someone who wants isolation either for themselves or for those around them.
I also plan to use these as reference headphones to check equalisation on field recordings and to listen back to binaural recordings which have to be heard on headphone for best effect, though I'd have to do that back at home. In the field I'd either use ear canal headphones or something like the 280Pro. The coiled cable on those is perhaps a consideration for situations where cable snagging can be a problem such as office or mixing desk.
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I've now directly compared these with the 280. The 280 do actually have more prominent bass, whether due to the cavity or that's the way they are. Also the 280 reveals things like cymbal hits in "Fun Loving Criminals" differently, maybe with more brightness (and a little more hiss from the sound card). You can hear the sticks hitting with both, but on the 280 it's obviously there. This test was done off a PC sound card.
The 595 shines off my proper amplifier. I've not tried the 280 in that situation. The 595 is softer once you've found how to make it fit well, and sounds more 'open'. You're best if you can trying both before buying to see which of the two you like best. They're both good. They're different to each other and maybe suit different music. They both reveal things that my old ones didn't.