Xonar DX guys definately supports hardware mixing but the important point here I d' like to mention is that most of the XONAR cards are working via the snd-virtuoso driver and NOT the snd-oxygen!!
Xonar DX is one of them so be careful not to load the snd-oxygen with a DX card and always read the Matrix at the alsa website to find out which driver drives your card.
I am making this note cause at openSUSE 11.3 distro still the Xonar DX card is connected with the snd-oxygen driver when you try to auto setup it through YaST and as a result it will never work.
So please when you mention the CMI-8788 driver mention both its modules not only the snd-oxygen cause users might be confused.
Now about Creative labs I must say that they have dissapointed me since the Audigy 1 era with their policies... No need to buy any product from them.
Sorry to disappoint you but the CMI-8788 has NO hardware mixing capabilities at all. It simply is not present in the chip.
Also needs to be asked whether we're talking OSS3 or OSS4, here. (On that note, I wonder if Oracle has retained Hannu to work on Boomer...)
Honestly they need to boil down to one sound subsystem design. just make it really good. Windows has WDM which sucks until you use ASIO to rip the windows mixer features out of the system to reduce latency.
Once you do that, it works well, but its not like the windows deployments and WDM arch is problematic, its just heavy on the latency which makes recording a total pain in the ass.