Thanks for the feedback- I had missed Mojave's post re: turning off UAC. I have opted to do just that as the system is a dedicated HTPC and is a well controlled OS environment.
I also agree with Glynor that this is almost definitely a driver bug- I see the same priority lift using FOOBAR in admin mode/UAC off (and stutter in user mode/UAC on).
I'm guessing that apps like Sonar and Ableton call the ASIO driver via Windows Multimedia Class scheduler which might insulate them from it's rogue behaviour.
I've also tried every available driver release (including the Unified Xonar releases) to no avail.
In fact I have opted for a mix'n'match of ASIO and WASAPI using zones in MC19.
I've set the ASUS D2x to 48khz sample rate as most of the time I'm viewing video when I use the ASUS card and use WASAPI for this sample rate (with MC19 at normal priority) and then use zone switch to change to ASIO for higher or lower sample rate material (at which point MC19 goes Realtime priority).
I've also got an SPDIF connected ARCAM DAC for "proper" listening
These ASUS cards are real "curate's eggs", pretty good HW for the cost with terrible idiosyncratic SW and drivers. Should have done my pre-purchase research more thoroughly