Thank you both Brad and Joe.
The information you posted stimulated my mind and I immediately thought of something that could be a factor. I wanted to experiment and test an idea before posting and it took me until this evening to gather the time and go through the steps. I had not had my 2nd keyboard hooked up and it involved installing drivers and going into the onboard software to set it up as a controller.
Visualizing how the headphones are plugged into the computer I remembered that the Mac Mini has a powered speaker system with its own volume control. The headphone jack works with and without this speaker system's power turned on. When I use the headphones I have the power off so the On/Off light is off. This was probably instrumental in my ignoring the fact that I had this additional boost lending itself to the audio signal. Typically I operate my computers with most of the alerts muted. I keep the OS sound volume at a standard setting and would have unconsciously customized the headphone volume using the volume knob of the speaker on my desk where the headphones are plugged in. I would not have had any occasion to measure how loud the system alerts were compared with the DAW plugin because the alerts were never triggered. This explains why I had been able to play my piano at an enjoyable level back when I was playing regularly a couple of years ago without feeling the audio signal was considerably weaker than it should be.
I set up my Mac Mini the way I had back when I was playing the piano regularly. I duplicated the set up on my Mac Pro. I tried my other keyboard, a Yamaha MOX, with both systems. Then I tried the PX150 with both systems. 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.
I have not played my keyboards regularly for over two years. I may have only tinkered a little a couple of times during this period. I don't even really know how to "play" piano. I just noodle around but I had developed comfort and familiarity. The piano action of the PX150 feels awkward to my rusty hands but the MOX feels fine. I started out with the MOX and was ready for a piano action when I purchased the Casio so it was a well-timed transition. This current awkwardness has been a factor in my perception of the response I was experiencing which prompted me to start this thread.
In conclusion, the audio produced by a bare keyboard seems inadequate at default settings in the OS and software. The boost from the external speaker system is necessary to attain a comfortable experience. Without this boost from the external speaker system I have to set the OS audio to max and increase the software volume and adjust the velocity curve. This is a slippery slope leading to undesirable side effects. So I am just going to connect the speaker system to whichever computer I am playing through.