Well I set up the Yamaha (the neck was too flat and the frets needed a good polish, plus a lot of nuts and screws were a bit loose), fitted a set of D'Addario Pro Steels (which are nice and bright), put a new battery in it and took it to last night's rehearsal. It was just what he wanted and everyone else complemented him on the sound. So the slap bass sound search is over. The only slight issue is that on the 'slap' songs he normally uses a dropped D tuning, but I said that he could just leave it set up with the E string tuned to D, rather than fit a Hipshot xtender key, though I've just proposed it to him as an option.