Anderton
I highly recommend de-installing ASIO4ALL as a test (you can always re-install it). I have run into interfaces where just having ASIO4ALL installed on the machine interferes with an interface's native drivers. The symptoms are the same as you describe - an inability to get decent latency settings without crackles and pops. As soon as I removed ASIO4ALL (de-selecting it is not enough), the interfaces were able to achieve low latencies and no crackling.

Yep, you better believe that the first thing I did before installing the drivers for the NI Audio 6, is to uninstall the ASIO4All I had previously used. I did not want a conflict either.

I also have NIKA6 and satisfied by it, no clicks and crackles when using ASIO.
If you experience clicks when moving volume sliders, this can be connected with your video card drivers (I had similar problem on another system back in the days). Try to update your video drivers or fully remove them and see if this changes anything.

@Bort - the idea behind my suggestion of you testing with DPC Latency Checker with Sonar NOT running, is to see if there are things outside of Sonar, but present on your system, that are interfering with the kinds of processing (streaming audio) that Sonar does.
 
If issues are found outside of Sonar, it means you have some things to iron out in your system, independently of Sonar.
 
Bob Bone