So here I am updating.
The seller happened to send me the wrong DT880 version, 250 ohm.
I noticed it first thing, because I cannot get enough volume (loudness) at 40% of the computer volume with Xonar STX. Then I switched to high gain (middle level). It got loud enough and then I went extra gain (highest level) thinking that it was the 600 ohm, and I could not bear the loudness. It went really harsh even at the volume 10%.
Out of curiosity, I checked the plug- no surprise, no 600ohm mark, and then I checked with multimeter at 2K scale, to read 0.236 and 0.237. It is confirmed, the seller send me the wrong thing.
Today, the replacement came for free.... Luckily I got a good ebay seller. And just to check the multimeter showed .587 and .589 left and right. Plug in at normal gain- not much detail, same with high gain. The volume right now is at 60% Xonar STX extra gain (highest level). Also, the 600 ohm mark on the plug is present. YAY!!!!
So impression on DT 880 600 vs 250, I have to say that with 600, I got more detail and the firmness of the sound is there. It is similar to jello or pudding or something like a jelly. At 600 I can say that the jello is crisp and firm while the 250 ohm feels more like quite flimsy and about to break to me. Instrument sound wise, I think they are quite similar, however the main difference is in vocal in my opinion. Main vocal may not seem so different among the two, but little vocal such as those background vocal in TSFH or audiomachine songs really have that crispiness difference among the two. Another song that is my benchmark is the opening of the song "Always" by Bon Jovi. The punch of the drums and piano are more apparent and FIRM in the 600 version than the 250 version. By the way, the comparison is between new, right OUT OF THE BOX, not burned in headphones.
Comparing DT880 vs Senn HD598 however I have to say that the sound signature is so much different. The HD598 is really flat compared to the DT880 (both version). It is not that I did not enjoy it, but it is just not what I was looking for. If people are thinking the difference between HD558 vs the rest I mentioned above, I can say that the sound quality is not a match. DT 880 and HD598 are still above HD558 (even with foam mod). I can not say WAY above HD 558 because the price range among the headphones that I owed are the same, not anything >$300 and such, but I enjoy listening to HD 598 and DT880 much much more than to HD558.
I am really considering to upgrade my amp. Forums suggested that even cheap beginner amp like Schiit Magni will improve DT880 600 ohm. I am totally like the way it sound currently, just with my Xonar STX, but that is because I have not been exposed to something much better (if there is, perhaps with tube amp). I may be considering selling my STX, get a DX for just its DAC and get a Bellari HA 540 amp. My experience with Xonar STX so far is great. No hissing, totally controllable, no error. My Fiio E10 has slight hissing sound at high gain unfortunately.
What is your opinion guys?