Low audio output but strong input connected to Mac

When I connect a keyboard to a computer the audio volume from the keyboard is significantly lower than any other audio source in the computer. This is happening with three separate computer systems and two different keyboards.

I have a digital piano (Casio PX150) connected to my Mac using a USB/MIDI cable. I am using Logic Pro 9 or Garageband for plugin sounds but the volume is significantly low even at the computer's maximum volume setting. It also requires impractical key force to generate volume. It produces a strong as should be expected signal according to the DAW meters but the actual audio I hear through headphones is weak. It seems like the default velocity settings are not sensitive enough but I really do not know enough to say whether it is not normal. From a practical standpoint this does not appear to be the way things should be by default.

It is impractical to accommodate this weak volume from a keyboard. I have a powered speaker system for computers which has a volume knob and enables me to achieve more volume bringing the keyboard up to usable listening and playing level but I have to carefully lower all other audio sources. When I record, all other source tracks have to be reduced to accommodate the keyboard track's low volume. Yet they register the same general input range in the DAW meters.

• The three computers are a Mac Mini running OS X 10.6.8, Mac Pro running OS X 10.10.5 and a Macbook Pro running OS X 10.11.3
  
• Keyboards: Casio PX150 and Yamaha MOX  (I have swapped while experimenting with identical results).
  
• Connection is with a USB MIDI cable (I have two which I have swapped while experimenting with identical results).
  
• Tried Logic Pro 9, Garageband , with their native plugins and Kontakt as a plugin as well as Kontakt in standalone mode)
  
• During experimentation tracks setup with Logic or Garageband loops played at normal volume.
  
• Quicktime and VLC video as well as iTunes files play normally and match the system alert volume while I perceive the keyboards as abnormally low in audio output. This means when I have the keyboard audio maxed out in order to use it I must turn everything else down or off in advance to prevent being blasted with volume if they are triggered.
  
• I have tried two sets of headphones (both are Sennheiser, the HD 580 and HD 600) it is not the headphones as I hear all other sounds at normal volume but I swapped them anyway with identical results.
  
• Using MIDI Meter I see the velocity range. The keyboards produce identical MIDI information.  Generally, with an average suppression of a key they register between 45-63. Bringing a key down hard ranges mostly in the 80's with as much as a single 109.
  
• Kontakt has a virtual keyboard. Probably inaccurate and unreliable eye candy but worth mentioning. Like a physical acoustic piano, pressing a key close to the pivot produces a softer note and when pressed at the front edge it produces the loudest note. If anything can be gathered from this both keyboards only achieve midrange volume or less and I have never been able to reach beyond 109. It took a significant thump to get to 109 and I doubt anyone would ever play that hard.

Does anyone have any idea of what could be causing this?

Thank you in advance.