Alec has described exactly the process which I use, when I convert my KN7000 sequences to WMA or MP3 format. I use the 'normalise' function in my digital recording program (Goldwave in my case), to increase the overall volume level, after recording and before saving the file to hard disk.
The Panasonic recorder is just a basic program and as such, does not include any editing functions like those found in Audacity or Goldwave. You might like to try an alternative Digital Recorder program......

The reason that the Main Volume control has no effect on the levels shown in the Panasonic recorder, is that the outputs to the recorder do not pass through this control. The Main Volume control is within the Audio amplifier (Analog) circuitry and does not affect the digital signal, which is extracted before the Audio Amplifiers.
However, the 'APC/Sequencer' slider does affect the USB output, since it is associated with the digital circuitry. So for maximum output level, you should always position the slider at the top of its travel.