In terms of FR, the HD560s are quite good, but I feel like they compress the dynamics a bit. Probably wouldn't be the only pair I use (but I'd always check on multiple anyway), but they are hard to beat in terms of price vs performance. The Beyers can be good for treble analysis in particular, but I have a big head and they are just not comfortable for me. Still, those would be my two picks.
DT900 pro x. I think its relatively flat without EQ. With EQ the bass is godly.
Is its treble response more laid back than the DT990s? Those actually hurt my ears
I havent had DT990 but DT900 is generally considered more balanced. Less V shaped.
I mostly used it with Oratory EQ. I might have had issues with particularly sibilant tracks. Only relatively sibilant ones are okay.