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Slightly noticeable tremlike sound and trimpot Q

Well, No big issue, but I think My 212 hd130 is started sounding a little bit "tremlike" or something like That ..when im with the band, I dont hear it, but when Im soundchecking alone..could it be a tube ?
Another question..why is there 3 blue trimpots inside? 2 on mainboard, and one on driverboard
Thanks in advance