TTS-1 velocity handling... Newb

Greetings! I am very new to Sonar and soft synthesisers in general, and I need some advice.

I can see that TTS-1 responds to the Vel+ value in a strange way. It responds to the negative values, but not to the positive ones. That creates a lot of difficulties because some instruments do not stand out well. 

Is there a solution for this problem other than adjusting volume of other instruments down to make those that not loud enough sound louder? I used to do that with Vel+ when I used hardware MIDI synths.