If the level of "hiss" does not change with the gain controls and the same noise is present on all inputs, it is virtually certain the problem is not tubes. Both channels/equal noise also means the difficulty is very likely in the power supply, which feeds each channel equally.
Also be aware that some (however little) noise will be audible normally at zero gain unmuted.
Be sure you are hearing a "hiss" and not a "buzz" (which could indicate a ground problem). Tracking down ground and ground loop problems can be a royal pain in the low end.
If nothing helps, don't hesitate to get it checked out by a reliable technician. You can get reliable recommendations for one of these invaluable persons by asking right here on audiogum.
Good luck. It's a fine preamp.
Also be aware that some (however little) noise will be audible normally at zero gain unmuted.
Be sure you are hearing a "hiss" and not a "buzz" (which could indicate a ground problem). Tracking down ground and ground loop problems can be a royal pain in the low end.
If nothing helps, don't hesitate to get it checked out by a reliable technician. You can get reliable recommendations for one of these invaluable persons by asking right here on audiogum.
Good luck. It's a fine preamp.