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

That it was clear, I speak about what loudness of hissing, I have made gaugings:

The difference is below specified in noise level (hissing) between switched off L2 and
Included L2. Connection to M1.2 cable XLR and RCA.
Monoblocks M1.2 are included. Inclusion and deenergizing of CD of a player
Noise level does not influence. Distance a boundary speakers and a microphone 20 inch.

cable RCA cable XLR

16 000 Hz 8,4 dB 8,6 dB

8 000 Hz 11 dB 11,6 dB

4 000 Hz 6,1 dB 7 dB

2 000 Hz 4,6 dB 6,2 dB

1000 Hz 3 dB 6 dB

500 Hz 0 dB 2 dB

250 Hz 0,5 dB 2 dB
I tried to include L2 through the network conditioner,
Different cables - changes are not present.
The previous owner has replaced lamps 4 months ago.

Such noise level it is normal or L2 demands repair?