Confused about gain: phono + pre, or just phono?

My point is that if you "underload" the preamp (load it to 325mV max instead of 500mV max, IF 500mV is its sensitivity) you will get less S/N ratio out of the preamp (less input signal with the same noise floor of the preamp = smaller S/N ratio).
You say that KAB tailors the calculation for their OWN preamps, with 325mV input sensitivity.
This makes perfect sense and only proves my theory that if your preamp has 500mV input sensitivity, thus you should use an outboard phone stage with 66dB gain to raise the output voltage of your cart.
Thus apply the mere definition of dB to get the needed gain to raise your cart output voltage to the sensitivity level of the preamp you'll be using. That's all I'm saying.
There are two variables to use in the formula, the output of the cartridge AND the input sensitivity of the targeted preamp that will receive the output of the outboard phono stage.

Ok, I got it.

I guess the "magic number" then is the input sensitivity on the pre-amp. Which assumes that the manufacturer actually knows it. LOL.

FWIW, Robert Harley's post on this (here: http://www.avguide.com/blog/taking-the-guesswork-out-phonostage-gain) seems to indicate that it's 1v you should be targeting, not .5v nor .325v.

So, what is the deal with that number? And how do I found out what I have? Is "input sensitivity" called something else?

Usually 1V is the starting input sensitivity of power amps. The ones with loads of power will have higher numbers in here or else you won't get to turn your preamp's volume knob too much before being blasted out of your seat.
So I guess that it goes this way.
1. you have the cartridge and know from its technical specs what its output voltage is.
2. You have/intend to buy a preamp and you know from its technical specs what the line input sensitivity is. Usually 500mV (but I might be wrong here). This is not the sensitivity of the preamps' phono stage which you'll bypass if it already has one. You want to buy an outboard phono stage, right? Because either your preamp doesn't have one or the one it has doesn't satisfy you.
3. Now you use optimal_outboard_phono_stage_gain = 20*log(Vpreamp/Vcart) where Vpreamp is the line level input sensitivity of the preamp and Vcart is the output of the cartridge.
4. Of course it's not going to be the end of the world if you're a bit off, that's why I said "optimal_outboard_phono_stage_gain".
5. Take a look here: http://en.wikipedia.org/wiki/Line_level
Might see them MAX for consumer electronics at 0.447V

Mihaitaa -- very helpful, thanks. FWIW, I think you're right. Or rather, that (my understanding of) Harley is way off.

Not only will a .25 mV cartridge work with 62 db of gain, it has worked optimally in my experience with 2 different cartridges and two different phono stages, neither of which were KAB phono preamps, for me over the past 5-6 years.

My previous cartridge was an Ortofon MC 20 Super with an output of .25 mV and it worked perfectly with 62 db of gain into a 100 ohm load using a modified DB Systems phono preamp (modified to give 62 db of gain) into a line input on an Audiolab 8000A.

I've also used (and am currently using) Denon 103R's which are rated at .25 mV into that phono stage as well as an Aqvox phono stage into the Audiolab mentioned above as well as a Jungson JA 88C integrated.

Both of the 103R's I've used in the past 4 years have come with individual spec sheets indicating slightly higher output than normal specification. One with about .29 mV and the other had .30 mV.

My Aqvox has variable gain on the balanced inputs between 60 and 75 db and I've run both of the above Denon 103R's at the lowest gain setting. In other words 60 db. Going higher is as simple as turning the gain pots on the front of the unit and I've done it.

Those cartridges work best in my system with 60 db of gain, not any more.

Too much gain (and not enough gain, for that matter) is not a good thing.