I put my amp output into a Marshall Power Brake (attenuator). The amp sounds great normally but it's very loud and to get the "sweet spot" of tone and volume, I use the Power Brake on amps like my 1971 Marshall 50 watt head (which have no Master Volume). I'm not having any problems with the Marshall 50 watt head but with my 64 Fender Bassman, when ever I turn the dial down on the attenuator to damp down the output, I hear a high pitch sound (not super loud but annoying as all heck). The Fender wants a 4 ohm load as this was the value of the cabinet that originally came with those 64 Bassmans. The Marshall Power Brake only has two setting: 8 ohm and 16 ohm. I'm sure I've used the Fender with the Power Brake before with success but I am guessing since I changed out the tubes and tweaked the bias to the optimal value for those tube, something - possibly a pre-amp tube, is causing the problem?