Oh, I see. As far as I know this does not exist. You will have to manually equip each instrument. If this somehow does exist, someone please enlighten me. I can't believe I cannot remember, but I believe the song checks for your instrument mod every tick and uses the highest one. If you equip a drum and cast a dot, then it will do damage per tick depending on your mod at each tick, but maybe it gives you the highest bonus when you cast it and it lasts for the duration of the song.
All of that being said, I only see this being super useful while in groups and changing out your weapons for a drum to make your DoTs do more damage in combat. While you solo, you always want to be wielding your drum. The only other time I can see you using this command is switching in a lute while soloing if you cast a DD/AEDD.
Sorry I am not 100% certain though. Give Felyn a tell in game for any other bard questions.