- One of the options a bunch of people mentioned is, why are you playing half the set in Eb and the other half in E? Is that a deliberate decision by the Band Leader/Group, or is it by chance? The first option would be convincing the band to tune to Eb and treat all the Eb and E songs the same. (Or tune to E standard and pretend all the Eb songs are in E if the singer can hit the notes. The singer's range will determine which way you go.) Seems like the best option to me.
- There's the 2-bass option. 2 basses into something like a Morley ABY Mix so all you have to do is put down one, and pick up the other, flip the active channel, and start playing. No futzing with unplugging/plugging between songs. Of course it means hauling 2 basses around. Second best option IMHO. Also good option because new bass.
- There's the Hipshot D-Tuner if playing in a drop tuning instead of a standard will work for you.
- There's down-tuning to Eb standard and using a Capo for all the E Standard songs.
- There's digital pitch shifters that'll pitch shift your bass down a number of semitones. A half step won't sound too weird but it'll always sound ever so lightly like it's been digitally pitch shifted. Nobody in audience will care but you may have opinions about it.
- 5 strings are an option, although one you'll probably not pick because you already said you're not simpatico with 5ers. (Just listing options)
- There's playing the Eb songs an octave up. I can't imagine this being a
good option, but it's an option.
Pick your poison.