I have a few of the newer Markbass products including the LMII with Traveler speakers and the Mini CMD 121P combo. These are 'digital' power supply head designs. I noticed that in some rooms there's a fairly loud line hum coming from the amp/speaker. This occurs on a variety of bass guitars, but is only audible with the active electronics engaged and the gain is close to wide open (but this is the way I use my basses). It happens when I'm not touching any metal (strings, controls). When my hands are on the strings, there's virtually no hum. This reminds me of ground loop stuff you could eliminate easily on amps I had years ago, but there's no polarity switch on these new amps and the plug is grounded, so no hope of reversing it in the outlet. At this point, I've isolated the problem to...power strips! I've used the gear in a number of clubs and practice rooms. When I plug directly into the wall or through an extension cord, no hum. When I'm in a power strip with other devices plugged in as well, hum. By the way, passive pickups or preamps don't exhibit the problem. Are there some issues with digital or switch-mode power supplies and power strips that any of you are aware of?