The H3000 is a stereo effect like the other harmonizers. Channel 1 is left and channel 2 is right side (normally). As I'm not familiar with the setup of the H3000, on my Eclipse you go into the Levels setting parameters and can change the input/output type from Stereo to Mono. You also are able to change the in/out dB levels to match the device you are looping with. Since you're utilizing a mono loop, you need to set your H3000 up to run Mono. You may also need to adjust the in/out levels to match your Marshall's fx loop. If it sounds distorted (when you're on a clean channel), turn your levels down on the H3000 (on my Eclipse I have it set to -20dB on the input side and 0dB on the output). I should let you know that I'm running my Eclipse in a rack that's "looped" with a guitar preamp that has a stereo fx loop, so I'm running mine in stereo mode, and I'm matching the signal level of the preamp I'm utilizing. The reason you only have one side of the signal bars lighting up is that since the unit is in stereo mode, it's only seeing that one channel, thus it's only signaling that one channel.
