The Sundara seems to be a bit bass shy, which is why I didn’t recommend it…
https://headfonics.com/hifiman-sundara-closed-back-review/
Here’s a review of the R9 by the same chap who reviewed the R10D and he makes several helpful comparisons.
https://headfonics.com/hifiman-he-r9-review/
The R9 seems to be even more bass heavy and colored with less treble detail — sounds like they were shooting for more of a Beats sound than a more neutral and linear audiophile tuning. Depends what you’re looking for.