Measuring the actual Muffsy Phono stage reveals a very good RIAA compliance, with a little extra amplification of the bass.
THIS ARTICLE HAS BEEN UPDATED WITH IMPROVED MEASUREMENTS.
The specifications for the Muffsy Phono Preamp PP-3 states that the RIAA compliance is a miniscule +/-0.025 dB. That's of course with filter resistors and capacitors measuring exactly as they are supposed to. Problem is, they almost never do.
The resistors have a tolerance of +/- 1%, which means that a 16k resistor can measure between 15840 and 16160 ohms at extremes. The capacitors have a tolerance of 5%, which allows for an even greater variance.
So what happens then, if you just solder in the components in the kit? I did just that, and measured the RIAA compliance from 20 to 40.000 Hz. Here's the result:
What happened is that the circuit measures +0.21/-0.16 dB across the audio band. At 40 kHz it's still only 0.21 dB below the ideal value.
Not exactly the +/-0.025 dB we expected, but that would only have been the case with precisely measured and matched components. The result is very good though, and on par with, or better than the best commercially available phono stages.
It's also worth noting that most vinyl records aren't following the RIAA curve exactly anyway. The RIAA equalization standard is what we have to aim for though.
Your amplifier and (especially) your phono cartridge and loudspeakers can be expected to perform way worse than this, as they are specified at +/- 3 dB across the audio band. This range happens to be the extreme values of the graph above. As you can see, the response is very close to being centered at the 0 dB value in the middle.