Davide256, you missed one- design of the phono section can affect ticks and pops quite a lot.
Bdunn, the first thing to do is to determine the value that you need. There are several ways to do that, you could get a stereo potentiometer, wire it as a rheostat, put that across the inputs of the phono section and dial it in. Radio Shack makes a 100K stereo potentiometer that would work nice for that.
Or you could start with a resistor value and see what effect it has. This can be a little trickier as you have to change the resistor to change the loading. That's why I like to use the pot.
But since you have a Grado, I already know that most of them like something in the 8-12K range. Your input resistance of the preamp is 47K, which has to be taken into account. So I would get a pair of resistors that are about 17-20K to use as loads.
Now some preamps don't allow you to do this easily and others do. Its been my experience that you don't want to interrupt the phono cable on its way to the preamp with an additional connector, but that would be one way to do it- get an RCA to RCA adapter and install the resistor inside it. Otherwise you might have to install the resistor across the input connection of the preamp. Of the two the latter is preferred as long as the installation isn't difficult.