Playing with Cartirdge loading on my phono stage

@holmz That is going to depend on how the phono section reacts to the RFI and IME that depends largely on the circuit itself so a blanket answer will be inaccurate. If it were me I would be looking at all those things. Its also worth noting that the electrical resonance may not always be in excitation- so it will come and go in certain circumstances.

does that mean we can just select the load with an o-scope?
and select the highest resistant value possible?

It would be better to use a distortion analyzer, but this would only be of benefit if the phono section can't deal with the RFI on its own. If the phono section does deal with the RFI properly, you'll see no benefit from the loading in terms of distortion.

Would it not depend on the frequency response of your speakers in your listening room with your tastes for sound in mind?

No. The reason the sound changes is due to distortion- LOMC cartridges and the tonearm cable together form an electrical resonance that generates Radio Frequency Interference which is injected directly into the phono section and can cause distortion. The loading resistor detunes the electrical resonance, thus eliminating the RFI.

However if your preamp was designed by someone that knows about this phenomena, you'll find that the highest load impedance (47,000 Ohms) is the right one, with little or no effect at lower resistances other than the tracking ability is reduced.