Hamelin's Tuner As the picture shows, it is an accessory that will let you play any song without going off-key if you use the Playing Instruments skill. This means your songs will always sound like they sound on the previews on MabiBeats or in 3MLE and that you will never get erroneous notes, however you can still fail a song if you attempt playing a Rank 1 Score when you have Rank 9 Playing Instruments for example, resulting in the score stopping abruptly and a message saying you need to practice more before you can play this score. The last bit is pertinent to your question. Musical Knowledge's Rank is only relevant to how you train Playing Instrument, IE: It's good to keep it one or two ranks above your Playing Instruments Rank to make sure you get all the training you can. However, once you have reached Rank 1 Playing Instrument, the matter of you having Rank 1 or not will not be a significant of your success. Many players do not know of this but Playing Instrument's Success Rate is also affected by Intelligence, which is largely given by Composing and Musical Knowledge. While this isn't a major factor, if both Intelligence and Musical Knowledge are high enough, you should have no problem playing your scores. In a Jam Session setting it is recommended that you keep it on a lower rank however because for one it opens the way for more bards to assist you with your jam sessions, and for two, it makes it easier for you as well. My advice would be to rank it and then lower it back down after you've completed Playing Instruments if you plan on doing a lot of Jam Sessions, otherwise I would leave rank as it is one of the higher INT providing skills in the game.