Dear Tobes: I heard the A90 in my system loaded at 100 Ohms and I don't try it at 47 Ohms.
I can't say if at 47 Ohms or at 100 Ohms has the flatest frequency response, like Pseydor assume, because I don't measured it and I can't calculate either with out the cartridge inductance value.
What I can to know is that passing from 100 Ohms to 47 Ohms the SPL goes down almost 1db and this difference in SPL ( IMHO ) is the one that could makes that we hear " differences " other that a change in the frequency response due that the A90 internal resistance is so low ( 4 Ohms. ).
My thought is that the quality performance does not change but only the SPL. So to evaluate ( other than make the measurements in real time. ) what we are hearing we have to even the Phono stage/line stage output/volume at 47 Ohms and 100 Ohms.
We have to think that our ears/brain are extremely sensitive to tiny/small cganhes on SPL. Many times when we are hearing a recording with two different SPL: normally we like the one that has higher SPL.
In the other side, Pseydor say: " They also become fatiguing after awhile. ", this is correct because at 100 Ohms there is almost 1 db of higher SPL but not ( IMHO ) because is a quality differences other that the higher volume that can affect our perception " after awhile ".
Around 1 db on volume difference makes that the whole system performance change too.
Perhaps a little more important subject about is the input phono stage resistor quality where we are loading that cartridge.
Anyway, if that load impedance is the one value that works in your system that's fine because that value is the one that gives to you music enjoying.
Regards and enjoy the music,
Raul.