You can download new drum kits, some of them free. There are many options to improve the drums on Korg.
The PA4X drums are improved over the 3X, so I haven't found it necessary to try to change them further. I changed kits on several styles on the 3X when I had one. All you do is open the Volume page, touch the drum icon twice, and select and audition some of the other kits to see if they sound better on the particular style. You can then save the changes to the Style, using the drop-down menu, or save to a Songbook entry. The Korgs retain both the changed version and the original version if you save to the Style.
There are also new sets included with the OS update.
I also stumbled on some new bass sounds that are very usable.
P.S. I just noticed Socrates has posted a free kit feature the Linn Drums sounds. You can download that and see if you still like them!
Thanks Soc-Man!