Personally, I would recommend not using built in live MP3 transposition, especially downwards or slower (or both!). The quality compared to doing it offline with a good computer program is quite night and day.

If I anticipate that I might need to do a song in an alternate key (horn player friendly or a step lower for high keys that work me a bit hard in case I’m ragged out!), I’ll prepare a transposed MP3 in the computer using a high quality transposition program (or even better, prepare a new track from the multitrack files I use with only the pitched tracks transposed and the drums and percussion left at pitch) and label it SONG-1.MP3, SONG-2.MP3 etc.

If you ever play these pre-transposed tracks next to one the keyboard is transposing live, the huge improvement in quality will quickly become apparent… You’d be amazed!

It’s a pretty simple step when first preparing a track to run it through your computer’s transposition software. Might as well make it a habit on tracks you think might need it in the future.