Okay, after a long time, I've decided to give the chance to change stock vinyl pads, and exchange them for AKG K240 Velour pads.
Oh my... what an improvement.
They are MUCH more comfortable. In terms of comfort, this upgrade is A MUST, and I should did it much earlier.
Now to the sound. It has improved a sound. I am still analyzing it, but from overall feeling, I feel it... the sound experience with this velour pads is remarkable better.
I think, the headphones are now more open. The sound is more smooth. The soundstage is little bit bigger. (can be because the AKG earpads are not so shallow as stock ones) And, to the last, it helped a bass too. Not in quantity. In quantity, basses are same as before. But in quality... they are nicer, than before. I am sure, that what I hear now, is better, than what I've heard before. I had these headphones with stock earpads 4 months, and used them very often, so I've got used to their sound, and for sure, I'm feeling improvement.
Many people waiting to buy these velour pads, because they cost almost as these headphones. But you cannot think this way. For me, headphones costed 25 euro, and velour pads 20 euro.
I am MUCH more content with modified SR850 for 45 euro, than with original one for 25 euro. Comfort do really much, and that smoother sound, and nicer basses, are worth the money.
With stock earpads, I was quite content with headphones, but still...in their sound, something didn't like on them, could not find exactly what. Also there were comfort issue.
But now... I love them. It is worth 45 euro, in every cent spent on them. I would say, with velour pads, they are overall like 75% of HD598. (if I don't count soundstage, which is remarkable bigger on HD598)
Next step would be to upgrade the headband... it's another weak point of these headphones. Some kind of soft foam would be needed to bottom part of headband.