Hi Kenji
When I first received my preamp, the same problem happened with me... I systematically tried all combination (almost I think). Finally, I turned off the power and took out all the tubes, reinserted them and turned on the power. After a few moves and when I toggled a top gain switch to High, there was a loud POP and faint sound was heard. Next I toggled the other top gain switch to High. Luckily, my speakers were not damaged (I think). For my setup, I had to switch off the power amp everytime the top gain switch is toggered, either way. Both gain TOP switches had to be set to High and back gain switch set to middle before I hear anything decent. You may try that.
Good luck, Edmund