I've got a pre out and pwr in on my fender amp. Sending the signal from the preamp to the headphones or computer is not a problem, but I'm not sure how to temporarily shut off the amp's speaker. I've tried shoving a heaphone jack into the pwr in input, but I'm guessing it's not good for the amp, since sometimes this causes a pop and I've had the amp serviced once because of a bad preamp-to-speaker connection. Is this what caused it? If so, is there anyway to keep doing this without damaging the amp? Thanks.