impedance is what effects the sound
too low = dull. too high = too bright
As a general rule of thumb that is often true in the case of low output moving coil cartridges. But the effect is also dependent on the phono stage, and for that matter on the capacitance of the cable connecting the output of the SUT to the preamp. (The shorter that cable is and the lower its capacitance per unit length is the better, assuming accuracy is the goal).
.3mv cartridge x 21.5 = 6.45mv = TOO HIGH SIGNAL STRENGTH
Nope. See below.
.3mv x 10 is 3mv, TOO LOW? (mx100z phono sensitivity is 3mv)
.3mv x 20 is 6mv" TOO HIGH?
I PICKED THE WRONG DAMNNN CARTRIDGE!!!
While sensitivity specs for power amplifiers can be useful, I suggest that you completely ignore such numbers in the case of preamps. All it means in the case of a preamp is what I described in my initial post, and pretty much all it is useful for is calculating the gains of the phono and line sections of the preamp (if they are not explicitly specified), as I did in that post.
A 20x (26 db) SUT will boost the rated output of the 0.3 mv cartridge to 6 mv, as you indicated. 6 mv is only about 1.6 db greater than the 5 mv ratings of two of your present MM cartridges. For a single-turn rotary volume control (as is provided on your MX110Z) a 1.6 db change in volume typically corresponds to around 10 degrees of rotation, and perhaps even less. So with the new cartridge and a 20x SUT you would simply be setting the volume control a tiny bit lower than the settings you presently use with those two cartridges, to achieve the same volume. (This assumes that the specs are reasonably accurate, of course).
Regards,
-- Al