Greeting--let me preface by stating that I have researched previous threads regarding ground-induced hum and that my problem seems to be a little bit different (though correct me if I'm wrong as my knowledge/skill level is modest at best). I just finished and installed the Speedball upgrade after using my Crack for nearly one year. Prior to the upgrade, my Crack was pretty quiet already, and the minimal noise was not of the 60 hz variety. My unit is plugged into a multi-plug power line conditioner that is,in turn, plugged into an isolated, dedicated 20 amp line with nothing else on it. With the Speedball installed, I experience a definite 60 hz hum upon tube warmup that does not vary with the volume control. The hum level does vary from outlet to outlet,however, ranging from mildly objectionable (only one particular outlet) to extremely objectionable (everywhere else). In my experimentation, found that by using a cheater plug on the power cord, the hum disappears completely. I am wondering if this is safe, though, and welcome any input or advice regarding my issue. I should add that I did go back over my Speedball construction and re-soldered anything that looked the least bit shaky,to no avail. All of my voltage readings are consistently about 10% higher than as specified in the manual. I really love the sound of my Crack and even more so with the Speedball mod, using HD-600s, but can't tolerate the hum. Please help!