Well by raising the Musical Knowledge of a scroll it makes it much easier to play if you're playing is the same rank or higher. Otherwise it gets more difficult. So a rank 9 scroll vs a r1 Musical Knowledge scroll, If you gave both to a rank 8 Playing instrument Bard They would be able to play r8 Easier it wouldn't be as difficult as a r1 scroll. While if you gave the scrolls to a rank 1 playing instruments bard. Even though they're at cap the success rate is higher for them on the r1 scroll then it is on the r8. So, while your Musical Knowledge should really only be as high as you need it to be. If everyone who has the Jam action is around r9 then r7 might be a little to high for them, while if the people who're able to jam are r5+ then try to get a higher Musical Knowledge. Really this skill rank either makes or breaks