Bart is a wizard. If he touches something, it starts working: a guitar, computer, band... you name it. Not only is he a wizard, he's also a gifted multi instrumentalist.
Jeroen is a musician on a cellular level. No instructions needed. He'll know where to go, what to do and how to do it. He never learned about music. He absorbed everything about it. He's also known under the alias All-Jazz-Era. A DJ of the better kind.