Pepp111, welcome to my nightmare. Brand new pre SP16L and 4BSST. POP when mute. POP when volume up/ from 0 to 4 on the front of the pre digital read-out. I upgraded all my ICs, speaker cables, surge protector... and still POP. I will have to send my pre across the country to have them technically check it out. I will be interested to what you will find out. Most audiophile suggested leaky-cap or bad tubes...but I run out of money upgrading things not neccessary and I run out of patience. Best luck.
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