The signal to noise ratio (SNR) compares the level of the audio signal to the background noise. An SNR of at least 65 dB is what's needed to have a completely silent phono stage. The Muffsy Phono Preamp's SNR, with OPA2134, comes in at 79.46 dB as shown below.
The measurement of signal to noise ratio can be a rather arbitrary affair. Some measure it as the difference between the noise and when the signal has 1% total harmonic distortion, while others compare it to a 0 dBV output. I've seen one manufacturer use 3 volts output as the reference point. With its 36 dB gain, it barely outputs a tenth of that with a normal MM cartridge connected.
For the Muffsy Phono Preamp, you'll get the signal to noise ratio as it is under normal use. This means that it will be referenced to an input of 5 mV, which is normal for a standard moving magnet cartridge. The output, at maximum gain, is then -2.54 dBV.
The measurement above shows the noise profile of the Muffsy PP-3 with OPA2134 operational amplifiers. The inputs are shorted in order to avoid picking up environmental noise, and the measurements are A-weighted. The graph is an average of 20 samples, instead of just picking "the best measurement".
The red square shows the total noise power, from 20 to 20.000 Hz, which is -82.0 dBV. The difference between the signal and the noise is then 82.0 dB - 2.54 dB. This gives a signal to noise ratio of 79.46 dB.
This is comparable to how commercial vendors measure the signal to noise ratio, but here you can be sure that it is under normal operating conditions. That said, your cartridge will also create some noise of its own. When you connect your turntable to the Muffsy Phono Preamp, the actual signal to noise ratio will probably be around 77 dB.
It's worth noting that these measurements were done with the Muffsy Phono Stage powered by the Muffsy Power Supply. Both were housed in a B0905 enclosure with the Muffsy Back Panel, as described in the build instructions. The components were taken straight out of a Muffsy kit without any matching of any kind.