As a rule of thumb, the input resistance of the phono stage should be about 10X the internal resistance of the phono cartridge, to assure a flat response from 20Hz to 20kHz, all other things being equal. So, first you have to know the internal resistance of the phono cartridge. For a moving coil, this will be the resistance across each of the two coils of wire that produce L and R channel signals. The manufacturer will or should publish such data. Measuring it yourself can be dangerous to the coils, because meters put a current across the coil that can burn it up if the wires are delicate, which they usually are. Technically, "impedance" is different from resistance, but for purposes of the match between MC cartridge and MC phono input, the two terms can be used interchangeably, although it’s true that the phono cable and the gain device at the phono input (whether tube or transistor) will have an input capacitance that also adds to the capacitance seen by the cartridge. (Impedance is defined as resistance dependent upon frequency. Capacitance adds to resistance at any frequency, to affect impedance at that frequency.) MC cartridges are relatively insensitive to low levels of capacitance (e.g., anything at or below 150pF, but this is not a hard number), because their inductance is very very low compared to MM cartridges (more than 1000X lower, in fact).
So, for a LOMC cartridge, the internal resistance will typically be in the range of 10 ohms, unless you’re dealing with a Denon 103 or others like it that have a relatively high internal resistance of 40 ohms (as I recall). If your LOMC has internal resistance of 10 ohms, then obviously your phono stage can be loaded with a 100 ohm resistor or any higher value, and you can expect this match will not roll off frequency response, much, below 20kHz. Still, some of us have found that loading an LOMC at 1000 ohms or even 47K ohms, for two examples, can yield a more open sound. A salient point is that the load resistor actually is loading the phono stage, so if your phono stage is very stable, 47K ohms works fine. If you are using a high resistance LOMC, like the Denon 103, then obviously you are going to experience some high frequency roll-off if you run it into 100 ohms. Some guys like this effect. As Chakster and others have said, you also lose some signal voltage when you get down below a 10X ratio between input (phono) R and output (cartridge) R.
Chakster mentioned "critical damping". As I understand it, critical damping occurs at the particular load resistance where you first start to lose signal voltage to ground; the idea is to go down only to the point where that phenomenon just starts to occur. You would need test instruments to achieve that goal accurately. I don’t agree with someone who wrote that you can use the load resistor as a sort of tone control by ranging it from say 100 ohms to 47K ohms. In my experience, once you get above the 10X ratio, there is not a great deal of difference to be heard as you approach 47K, except a very subtle sensation (in my system) of greater open-ness to the sound. It’s subjective, but if you were measuring frequency response, you wouldn’t see a great deal of difference.
