Here's the technical deal. "It's complicated," as they say on facebook ...
In the old days (i.e. before digital) recording studios, radio stations, etc. used VU meters. Those meters showed the signal level averaged over something like 300 milliseconds. This reflected the perceived sound level fairly well. The instantaneous peak voltage, however, would be much larger - on good recordings of unamplified instrumental music, the instantaneous peak signal is about 14dB greater than the peak VU measurement. This difference was called "headroom." For tape and vinyl, the 0VU reference level was chosen to allow that headroom, and used to specify sensitivity for amplifiers etc. It was assumed that the device would handle the headroom gracefully.
Digital recording, however, has very hard limiting, and the standard level is the clipping level - in other words, there is zero headroom. Sometimes you will see dB(FS) indicating full scale. The 2vRMS specification for CD players is not really comparable to the 5cm/sec vinyl signal specification. If you look up measurements of cartridge overload, you will see that most cartridges will handle 25cm/sec signals, at least in the midrange, and some as much as 50cm/sec. That's a headroom of 14 to 20dB.
The bottom line is that a a phono preamp with 40dB gain (a factor of 100) will amplify a 5mV signal to 500mV, with instantaneous peaks of 2.5v. A standard CD, specified at 2.0v, cannot put out more than 2.0v.
It gets more complicated, especially when you get into highly compressed pop music, which may have as little as 1.5dB headroom. But I don't want to go there ...
