Man, vinyl...when does the tweaking end...
So while waiting for someone that knows stuff to answer, I looked up the MM cartridge that I should be using and it said
"Output voltage: 4.0 mV (1kHz, 5 cm/sec.)"
So 2mV is half the voltage, 6db, I think we're reading the same book there.
Maybe Hana make that suggestion with a broader perspective, since not all phono preamps are as flexible in their gain setting? OTOH, that also gives you a chance to put the volume knob closer to maximum. I would guess it's better to bring up the signal inside the phono rather than inside the preamp since the phono is expecting teeny tiny voltages and probably liquidates noise there and then--eg my scope was a lot happier measuring phono out than phono in.