Somewhat related to this problem, I have noticed that when playing back a sequence, whilst viewing the normal sequencer screen, if I hit either the 'EXIT' or 'PLAY' button to display the Home screen, the APC volume sometimes increases.
There is a tie-up between the physical APC/Sequencer Volume control and the APC 'Slider' control on the Home screen. If you move the Physical Volume control, you will see the Home screen APC 'Slider' also moves. This would suggest to me, that unlike a normal volume control, the APC/Sequencer control is not just an audio fader - but a type of digital control into one of the microprocessors in the KN. So it may be that some kind of electrical disturbance could result in the APC volume changing. It seems that APC volume is determined by the last of these two controls, to be moved. If the Physical volume is set to maximum and then the APC Home screen slider set to say about 50, start a style playing and then carefully reduce the Physical volume and you will hear a sudden jump to the level determined by the Physical volume control. As Roger has pointed out, I always leave the Physical volume at maximum and control the APC level using the Home screen APC control. This setting can be stored in Panel Memory and may be recalled as normal. However, the Panel Memory must be in Expand Mode for this to work. Strangely enough, although there is an option in the Expand mode to turn on/off storing the accomp/Bass Volume, it makes no difference whether it is set on or off - as long as Expand mode is selected, the APC volume levels can be controlled within panel memories. Even if every parameter in the Expand mode is set to off, the result is the same - memory recall of APC volume works OK.
Incidentally, I also tried this on my KN6000 and it's just the same.
I would suggest that you try setting the Physical APC/Sequencer control to maximum and use the Home screen control for APC level and see if the problem still exists.
Just my two-pennyworth......

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Willum