Hi, I bought a pair of HD599 from
https://www.verkkokauppa.com/fi/product/65484/hfvhh/Sennheiser-HD-599-kuulokkeet
for €219,50, got it today.
First impression: sound quality is practically identical with HD598. Not at all approaching HD600 or anything like that. The bass might be just a bit more prominent in HD599 than in HD598; it's just noticeable, but probably this is due to this pair not having burned in properly yet? Also, comparing with my existing HD598, the burned-in 598 has just a slightly, just a tiny bit really, crispier sound, but I expect that the 599 will reach similar crispiness after the burn-in period.
So: definitely a bit more bassy at least at first, (so the sound is not immediately balanced like with HD598). And some crispiness (but not much, really) is lost at least straight out of the package. These differences are so subtle they might be matters of opinion, though. Writing this and listening to Davie Bowie's Width of a Circle, however, I notice now that I do automatically pay more attention to the bassline (because it really is more prominent) than listening to the same song with 598. Yes, indeed, the bass does jump out with this new model more. It might not be just a question of burning-in but of a conscious design choice? Remains to be seen. It's a good sounding bass, I really don't have enough experience of different headphones to say how tight it is, but it seems to be tight enough and controlled; at least not muddled and soft. It's the kind I am starting to like as we speak, though I am not a bass aficionado.
It is very comfortable to wear, a bit stiff still because new, but I expect it will stretch out with time. I can't unfortunately compare with HD598 in this respect because I had to put tape around the HD598 headband to keep it together, because the filling started to burst out some time ago (the seam cracked). So, side-by-side comparison is not possible for me with respect to wearing comfort. The earcups are exactly as big in both models, judging with bare eyes.