Sorry for any confusion, I'll try to make it clearer. You should have a Synth audio track for Kontakt which is where the audio comes out. You should also have a midi source track where you record the midi data feeding the Kontakt synth. On the Midi source track slide the volume in Sonar (just above pan and velocity) up to 127. You should see the Volume slider in Kontakt slide up to -6db. If it stays there on playback, great, problem solved. if it changes when you hit Play, then you may have automated the volume CC. If so, right click the volume envelope data and Delete envelope and try again. On the other hand, if you move the volume slider on the midi source track in Sonar up to 127 and the volume slider in Kontakt doesn't go all the way up to -6db, then you have likely entered 'Offset mode' and further offset the volume down. For example with the Sonar slider at 100 and the Offset mode Sonar volume slider at 80, Kontakt will snap to -18db just as you describe) To check Offset mode volume slider. Look at the Midi track Volume slider in Sonar, Press 'O' (as in The letter O) to enter Offset mode. When you press 'O' you will see Sonar's volume slider say Volume+. In this Offset mode the Volume+ should be set to 101. If it's not 101, set it to 101. Now press 'O' again. The Volume slider in Sonar should now say Volume (no + sign after the word Volume). This lets you know you are not in Offset mode anymore. Try sliding the Sonar Midi track volume slider up to 127. Again you should see the Kontakt slider slide up to -6db.
In recap, If there is no volume automation data on your midi source track, the midi source track volume slider in Sonar is set to 127 AND the Offset mode Volume+ slider in Sonar on the midi siurce track is set to 101, then the volume slider in Kontakt should remain at -6db. If not, double check all the stuff I just said, OR make sure you don't have other siurce tracks with different volumes pointing to that instance of Kontakt by mistake.
I find the -6db setting to work best as occaisionally I still have to lower the midi track volume to keep drums from hitting red within Kontakt.
Clear as mud? 😄 I hope I explained it better. I did test it on my computer using the same factory trumpet section for the test.