I have the wk-3000...but basicly the same for this question. Yes, you can run the drums, and split the keyboard...and play the bass part. You need to turn off the accompanyment...so you would be playing everything but the drums. The other option is to play along with SMF files you create (leaving out the melody and bass part)...that you add 'live' to the mix. Still another option is to use a rhythm pattern you design to play with...using the variation buttons to control the accompanyment ( too hard to explain here).
The volume level of the headphone jack output is adjustable and can be amped just fine. I use the headphone jack into a Roland Kc-300 amp, with no problems.