Just happened to look at the Ketron Forum.
Good question.
When I originally got an arranger (my first was an Orla module),I had assumed that I could play the bass in the same manner as when I played acoustic only and the arranger would follow - ie the sound would be an acoustic accordion backed by a "band".
Don't know if it is possible - at least I haven't figured it out! If the arranger is triggered by the bass buttons, you are correct, as soon as you play the counter bass or the 5th of the chord, the arranger assumes that you are on a new chord.
What if the arranger is triggered by the chord buttons? I use the key start/stop feature on the X4 so the arranger stops as soon as I stop pressing a left hand button. In this mode, if the arranger is triggerd by the chord buttons, it will stop playing when the bass button only is pressed. Since the usual way of playing the acoustic accordion is bass button/chord button/bass button/chord button, if you trigger the arranger with the chord button, the arranger won't start or change chords when you want it to if you play as you do acoustically. The accompaniment will also stop each time you stop pressing a chord button. What would be the result if you didn't use the key start/key stop feature?
If you don't use the key start/stop feature, the arranger stays on a chord until it gets the signal to change the chord and when you end the song you have to press stop. In this mode, if you triggered the arranger with the chord button, you could play an acoustic type bass pattern as long as you started the pattern by pressing BOTH the bass and chord button so the chord would start sounding. The chord would keep sounding when you resumed the acoustic pattern of bass button/chord button, etc. However, each time you hit the chord button, the accompaniment pattern would start again albeit on the same chord (accompaniment patterns can be as long as 8 measures before repeating, so thay sound less canned). The accompaniment ends up sounding very jerky, so this is really not satisfactory.
I have experimented with the "full keyboard" mode so the right and left hand of the accordion triggers the chord and wasn't pleased with the results. If I recall correctly, there were many times that the chord triggered wasn't correct.
So, when I play, I press both the bass and chord button and do not use the acoustic pattern. When I had the Excelsion MIDI Vox, I triggered the accompaniment with the bass or the chord button, but with the Millenium I can only trigger the acocmpaniment with the chord buttons. Haven't figured out how to trigger it with the bass buttons. I still press both bass and chord buttons because I am better able to tell where I am on the left side then if I only press the chord button.
Perhaps, the XD3 has a feature so what (I think) you want is possible. Experiment. If you come up with a solution, let me know.
All of the MIDI accordion players I have heard around here simply assign different sounds to the bass, chords and right hand and use the drum accompaniment only on the arranger module. So, they can play the normal acoustic bass pattern. The Chuck Berger video that petosa has demonstrates this very well. I prefer to use the full accompaniment.
Hope the above makes some sense. It sure took me along time to put it down.
Good luck!
RJM