Just hear the sound from these Siemens F2a mono-block amplifiers. After finish assembly these amplifiers. I've found the problem with "Hiss" sound and try to discover the main cause. Not from ground-loop or components but I missing "Grid Stopper" on WE409A/6AS6 the driver tubes. Problem's gone when install carbon composition resistors on their grids. Waiting for spray their cover part to complete the look. So my cat fell asleep with the sound from this amplifiers. ZZZzzz