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The higher difficulty the tournament quest, the higher the drop rate! That said, though, they're nonetheless very low. Since the event is close to ending and (iirc) Nero Fest never reruns —and instead has a different version each year, so Event CE's like Joint Recital won't be able to be reused for event bonuses— I'd suggest keeping your Joint Recitals un-merged after the first, keep the MLB on your best Support (after White Rose'd CE Support Servant), and burn/sell the copies after the event ends.

As for the exact drop rates (according to this site):
-Novice, 10 AP: 0.20%
-Intermediate, 20 AP: 0.40%
-Advanced, 30 AP: 0.80%
-Expert, 40 AP #1: 1.20%
-Hero, 40 AP #2: 1.60%
-Conqueror, 40 AP #3: 2.00%

As mentioned above, the drop rates are the best on the 40AP Hero and Conqueror nodes.

I managed to get some copies from the previous weeks' Hero and Conqueror nodes, and just got my 11th copy 2 days ago on the Conqueror node. Keep farming that and you'll get it eventually!