Resistive Loading RP-1 and Ania Pro

“Too low” would tend to sound dull, and you’d sense there was less gain. You’d want to turn up the volume a bit. That effect becomes increasingly noticeable as you reduce the input impedance in relation to the internal impedance of the cartridge.

Assuming "Ania Pro" is your cartridge, and that the recommendation is to load it at 100 ohms, and assuming "RP1" is a phono stage formerly set at 300 ohms input impedance and now set to 100 ohms input impedance, most likely it will make no significant difference.  But because you made a change, you will be inclined to hear a difference. If a cartridge is suitably loaded at 100 ohms, it will work nearly exactly the same at 300 ohms.  Manufacturers almost always state the recommended load impedance for a cartridge as a given number of ohms, or greater.