think future cartridges as well as your existing one
therefore the ideal is ability to change x factor separately from impedance. that will typically involve rear/internal whatever, expensive and things you don't like. BUT, if you want the best, it's worth the work. Do a temporary hookup, easy access, mess with settings, decide, then move equipment to designated location.
each cartridge: you do the work to get the best, then leave it alone, until? formulas guide you to what is 'predictably' best, then you narrow it down with your and your friend's ears.
your cartridge .3mv signal/5 ohm coil impedance: formula says coil impedance multiplied by 10, in your case 50 ohm impedance as a 'goal'. specs simply say greater than 30 ohms.
short of individual settings is a few pre-set optional x factors with the 'resultant' impedance that comes from that. that's what I went for. Front Controls!
In my case I use the MM Phono in my Tube Preamp, and a separate SUT (step up transformer) to boost any low signal strength up to MM signal strength, or PASS for any MM or MC High signal (bypass SUTs Transformer's windings.
I chose Fidelity Research FRT-4: 4 optional settings and Pass. SUTs are Passive, no power needed, no noise, you retain the sound quality of your MM Phono Stage.
It's 10 ohm setting yields an x factor of 20.68 and a 'resultant' impedance 110 ohms shown to 47k phono stage. before loss you would have a calculated 6.3mv signal strength.
An x-factor of 30 results in an impedance of 52; however that calculates to a 9.0 mv signal strength, possibly overloading your MM phono stage.
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Entre ET-100 also has 3 front controlled settings and PASS
both just happen to have 3 inputs for 3 tonearms which I need, and I change my various cartridges, listen to friend's cartridges, so the 4 optional settings are 'close' to what is needed. One with internal or rear switches would offer 'closer to ideal' for any current or future cartridge.
And, using any SUT, you need a MM Phono Stage somewhere.