The tribute band I'm in used to tune down a semitone to help the male singer. But one of the songs, the original band now play tuned down a tone so we also did it down a tone. However, that in the original key featured a bottom E, which became a D below E, which, as we were playing a semitone down, became an Eb, which I was trying to work out how to play. The bassist in the band that we're tributing simply used another bass tuned down a tone - my options were a D-tuner which would have been weird, a pitch shifter which would have worked at the cost of bass tone, or a spare bass. Then we got a female singer and put everything back to original keys, which helped an awful lot and meant I didn't have to either use my spare bass or justify the purchase of another Warwick (for authenticity). I suppose I could have used a five-string but the bassist in question doesn't.