I got HD595s from a friend for a while, I myself have the HD 25-1 II. I am using Asus Essence STX with better op amps.
I can only compare these two. These two cans are so different sounding you have to listen to believe it, cause it's both sennheisers, but are worlds apart, woooorlds apart. The stereo quality overall is much better on HD 25-1 with the sound textures that you can feel and touch rather, than hear and they are so close to you, it's really intimate, all in the head. The highs are better quality on HD25-1, the sound much more alive and bass, well after HD 25-1 the 595 like have no bass almost at all. The mids are great on both, still better on hd 25-1. What I don't like about 595 is the distant and a bit fatiguing on some sounds( for ex flute going up and down, chorus etc, I'm not sure how do you call it, hd 25-1 has no fatigue at all I guess it has a better extension), for me it is like I can't hear it well enough, it is thin as if unamped properly, but maybe it is just a mistake to compare hd 25-1s to hd 595s. The comfort in the 595 is perfect, you put them on the head and feel no discomfort at all, HD 25-1s are perfect as well, but they are pressing on the head and after too long use ears can hurt a tiny bit cause the pads are pressing on them all the time. I would pick HD 25-1s any day, any minute and second, they are just that much better, alive sounding with great punchy bass and phenomenal separation.