mV output from cartridge, please explain

If the cartridge were a Shure product very detailed specs are available on the Shure website. I don't know what your cartridge is so I don't know how they spec it.

Shure specs output as millivolts RMS, with sinusoidal groove modulation at 1000 Hz having a PEAK velocity of 5 cm/sec. Other manufacturers probably follow Shure's example.

Twl speaks of having the volume "wide open" as if this were a bad thing. Actually, the amplification circuits are always wide open, and the volume control cuts the ouput voltage down to the level you want. Ideally you would have just enough gain to fully drive the power amplifier with the volume control maxed out (which really means it is having minimum effect). Most rigs end up with the volume control between 12 and 2 o'clock when playing loud, which means that there is unnecessary gain in the system.

Shure put out a test record that had grooves cut at the full range of cm/sec. The purpose was to evaluate "trackability" of a phono pickup, a test which Shure pickups are good at. Few pickups will track the fastest grooves without audible breakup.

If you could get a hold of one of these records you could actually measure your preamp output voltage at the maximum groove velocity that your pickup can handle.