I asked M-Audio whether my MT is defect, but they say they would measure the same

so the MT is not defect - in their opinion - lol.
SNR = 20 log (Full Scale Signal Energy / RMS non-harmonic noise energy)
Since we are measuring a recorder, we have Full Scale Signal Energy = -0dB max.
The best-case szenario for a recorder or fwiw a preamplifier are shortcircuited inputs. Normally you would measure with a resistor like 200 Ohms to mimic your microphone, but that will only worsen the measured SNR, so I just short circuit the TRS input.
Gain is set to minimum (L / recording level indicators full left).
Then just record some seconds of silence.
Load in Audition, select the whole file do an "amplitude statistic". This will show you the average RMS Energy. The A-Weighted Value will be higher, but since the noise is nearly equally distributed over the audio frequency band, i fear that the A-weighting will not change -79dB to -100dB (A).
I'll write another email to m-audio, as i told you before, their reaction was "it's not defect, we measure the same". I would then imply that the whole series has the same problem.
If you want to do me a favor, short circuit your MTs inputs (TRS, of course w/o Phantom Power) and record some seconds, i'm pretty sure the outcome will be the same as with mine. IMHO the unit should exhibit the performance m-audio advertises. Here in the EU, the manufacturer is liable for defects of the unit for 2 years afaik.
I'll return it sooner or later. Just don't want to wait for 2 or more weeks to get another one with the same problems.
Tom