@intactaudio @lewm Thanks guys. The information is indeed very limited... all the website gives is this:
PHONO (MM): 2.5mV/47kΩ
PHONO (MC): 0.3mV/100Ω
Basically no information expect that the loading for the MC is very low, which explains why the Soundsmith Zephyr Star MIMC is not very happy. The sound is really wide and you can clearly place each performer in their space, but there's no highs at all.
I'm not much of an DIY guy and the prospect of opening up the L-590AXII to replace a resistor is not something I feel comfortable with. I sold all the separates to simplify things and having to modifying a new amp isn't very appealing. I think changing the cartridge is the easiest and best solution.