Hi Allan:

I'm not familiar with the KW 3800 but... if it has "arranger capability"... simply select the bass instrument (Yamaha calls them voices) that sounds best for the group's style of music... then turn off all other instruments, crank up the volume to where it needs to be... and play the keyboard chords needed to emulate a bass. It may take a little "fine-tuning" to reach the sweet spot desired but should be possible... given the "caveat" that the keyboard needs to have arranger capability or similar features.

Let us know if this does the trick.

Regards,

Dave Rice
Songwriter