Ive got an ESP LTD 5 string with about 15.5mm string spacing. Never played another bass as narrow as that. When I got it about 15 years ago, I do remember having lots of trouble slapping it, but I kept at it until it was natural. Now I am getting used to a Steinberger synapse with a 16.75mm string spacing. Even 1mm is very noticeable.
Then again I do play an electric double bass with 24mm spacing too.
Its all a matter of adjusting to a particular instrument.
Then again I do play an electric double bass with 24mm spacing too.
Its all a matter of adjusting to a particular instrument.

