I have been looking at the DT990 Pro's and HD598 going back and forth everyday, today is an HD598 type of day. So, I see a lot of people saying that the bass in the HD598 is not enough and others saying it's enough. I currently have a pair of Bose QC25's (imo, the bass is eh for me) and wanted to know if the HD598 had more bass or less bass, or would it distort in a song like Tropkillaz - HIDEHO (Bass Boosted) (the QC25 doesn't distort, problem is not too powerful, and the HD598 is open back so I think it might rattle/distort). So, yeah, thanks.

QC25 has more bass than the 598, below 70Hz. DT990 has more bass than the 598, but starts to fall behind the QC25 at around 40Hz. Depending on the kind of music you listen to, the lack of sub-bass may not be that noticeable to you. Also, if you find the bass on the QC25 not enough (I think it is just enough), you will have a hard time finding an open-back headphone that will outperform the QC25 in sub-bass.
 
I don't think you should worry about distortion/rattle with either the 598 or 990. They are decently built headphones. If their sound starts to fall apart, either your music is not mixed properly and has too much sub-bass, or that you are listening way too loud!