If you are using 2 interfaces for some reason you will need to make sure Sonar is able to use the one you want by disabling the one you don't want.

If you go to the 'Drivers' tab of the Audio options uncheck all the instances of the driver in use as if it is in Asio mode Sonar can only use one device at a time.

Therefore disabling the ASIO 4 ALL input and ouput drivers first will allow you to then select the KA6 devices Inputs and Outputs in ASIO mode.  You'll then be able to make a fair comparison between ASIO and WDM/KS (Kernel Streaming, not Kansas btw... )

You wont have to disable or uninstall anything outside of Sonar unless you want to.

For example I use my onboard card to monitor and external module and have the output of my interface set to be the 'Listen' port for that sometimes.  Also ASIO4ALL is often misunderstood and can be used for other things than just giving a cheap sound device ASIO ports it can also aggregate the WDM drivers with the native ASIO drivers allowing all sorts of 'loopback' recording options from various sound sources if it ever takes your fancy to do that.