I've got the 770 Pro (80 ohm), 990 Pro (250 and 600 ohm) and the new 900 Pro X. Compared to the 770, the 990 bass is reduced but there is still a bass boost and more so than most other open back 'audiophile' type headphones. In terms of the general sense of how 'bassy' the 900 Pro X sounds, without concentrating too much, I'd say it is broadly the same as the 990 Pro. As soon as you do start actively listening hen it is clear that the 900 Pro X is more defined/tighter in that region and it also extends deeper, so you get sub bass too. I'd got so used to my 990's that hearing the bass go so low was quite a a surprise. Not that the 990's are especially bad in that area, in fact they are like most other open backs. The 900 Pro X also isolate better than the 990 Pro, which makes me think that they are heading towards a semi-open design, which would also explain how they avoid as much low frequency roll-off.
In other ways, to me, the 900 Pro X differs from that generally V shaped signature that the 770 and 990 share. The mids are more projected and the high frequencies have definitely been rolled-off. Personally I like less of a treble roll-off and enjoy the V shaped sound, but I am confident that many will consider the 900's to be more accurate in those areas, whilst also enjoying the presence of some sub bass.