Hiss at addition in system LAMM L2

Hiss (noise) from speakers at addition in system LAMM L2.

Three weeks ago, I have bought preliminary amplifier L2 reference
Improvement in sounding all system was huge!!!! The overall dynamics and scale of the music!!!
Illusion of a presence(finding) in a concert hall has appeared.

But I am disturbed with the following:
At addition in system L2 strong enough noise (hiss) has appeared from
speakers. At inclusion of toggle-switch MUTE hissing disappears, at deenergizing
Toggle-switch MUTE all room is filled with noise (hiss).
Even at listening music on silent passes or piano music
Such noise level it is normal or L2 demands repair?