Hey, I suddenly feel very lost.
We need some kind of sampler/backtrack box/metronome (I don't know what it's called) that plays pre recorded stuff along with us live. For ex some piano parts, strings and stuff.. as you can see in many bands such as Evanescence and others that have synthesizer pieces but no keyboard player.
I've seen many bands, big and small, use this. Most often it's the drummer that handles that part. Of course it's very important in this situation to use a metronome that is "connected" with the back track. So... those of you that know.. what do you call such gear? What do people use?