From memory because i dont have the H9 Gen 3 at the moment, i found them to be more bassy, neater mids and the highs were almost the same. but sonically to me, it sounded a touch cleaner. you could easily make out detail in timber instruments. I found while listening to classical that certain instruments felt too sharp? but thoes were very rare to come across, mostly with violin and flute.How close are the H9 gen 3 and H6 sonically? Can talk about the differences? The H6 was my 1st portable can, and I chose and preferred it over the Momentums.
For more modern music, they were excellent for what they are, comfort, sound, build and ease of use were all top notice for this price bracket.
That being said, it is a more beefier B&O house sound. which is why i like it, however, i do prefer my H6Gen2 slightly more only because a) i own them already, b) cheaper, c) lighter.
But, if i had the money right now, i would 110% drop them on the B&O H9 Gen3 as my new portable headphone.