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?
This sounds like what I had when I owned a L2-ref preamp. Bought used off a dealer it exhibited the same behaviour as yours. Discussed the issue with the dealer who said some hiss was normal and he could hear some hiss sitting over 15' away from his set-up so my sitting 8' away would make sense that I would hear some as well. Mine was more predominant in the left channel. I bought new tubes, rebiased everything and still there was hiss. Swapped cables, rack location (maybe there was EMI/RFI issues?), removed preamp from rack to get it away from digital gear - you name it I tried it and still there was hiss. Drove me nuts enough that on Lamm's advice I sent in both the control unit and power supply - two 40lb. wooden crates overnight to Brooklyn. I had Lamm change out all the toggle switches at the same time as one was sticking, but I digress. Lamm checked it out on the bench and in their system and said all was OK. Got it back - overnight freight again - ouch and still the hiss was there. Loved the preamp in terms of what it did but couldn't take the hiss. Possibly it was a system synergy issue as the new owner mated it to Lamm amps and all is well.

And I do know what "normal" tube hiss should typically be as I've owned Rogue, ARC and Hovland preamps in the past and this was not the norm.

So maybe yours does require a check-up @ Lamm as it may not be the same thing that mine had or maybe its a synergy issue with your remaining gear (amp)?

BTW I was able to audition a LL2 deluxe and there was no problem with it and it was extremely enjoyable in my system.

Who had raised(increased) Hiss (noise) from speakers at addition in system LAMM L2?
My system:
Mark Levinson ML-390S
Preamplifiers LAMM L 2
amplifiers LAMM M 1.2
Dunlavey SC-V
Purist audio Venustas
Can bad lamps influence on strong Hiss?