After using ODAC+C421 with HD598 for over a week, I think HD598s are good headphones but not great. Of course, it is just a normal average headphone on the headphone map but still good value for the price. The only thing I guess HD598s can compete against other high-end headphones is the soundstage but unfortunately this is the weak point too. Why? Because when you plays some instrument songs with wide soundstage it is awesome but if you hear wide soundstage recording with the voices you can hear the voices are so far away. It looks like you are sitting at the second class seats not the first class seats on the plane and in this situation on the stage I think they are the same. And what about when we play some small soundstage recording songs, you may find out that the treble is hitting around you face, too too laid-back and it may causes you a headache. The result, you hope that your favourite songs would be recorded at an average soundstage not too small or not to large and it is really annoying.
There will be a Sennheiser meeting next week on Sunday in my country where Sennheiser will show IE800s and Momentums, of course with HD598s, HD600s, HD650s, HD700s and HD800s. I am going to join the meeting and have a chance to find out more about their combinations. Hopefully I will find a good combo for HD598 which may help me abit. If not the only way to solve my problem could be believing in Moon Audio Black Dragon V2 as good as it was mentioned in this review.
http://yklee118.blogspot.com/2012/03/sennheiser-hd598-live-update-round-two.html