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Ok, I've investigated.....
I found that I can turn the bass and percussion sounds off permanently in User mode, but not Preset mode.
(I sometimes copy Preset patterns then save them as User patterns. I then "tweak" them to suit my application.)
1. Select the User pattern you want to turn the bass/percussion off on.
2. Press and hold Erase, then press and hold Bass then Rec(Enter). The bass sound is then turned off.
Repeat for Percussion, obviously pressing the Perc button instead of the Bass button.
..... BUT.....
.... don't forget to save the modified pattern before you switch the SR-18 off!
3. Press and hold Save/Copy, then press Rec(Enter) to save it.