Of course, rules are there to be broken, but only he who knows them can ignore them. If you don´t know the impedance rule, you´ll twist the knob trying and guessing, but the 10x formula will give you a starting point.
If you want the "basic colour" for which your mic has been designed (and it´s not a coincide that manufaturers, on their specification sheets, sometimes recommend an impedance load), use the 10x thing. Once there, if you don´t like what you hear, are heading for a darker sound, a more open one.... Twist it, baby!

and aim for the sound in your mind. This way, you have, at least, a starting point. The other way round, you are just not knowing what to do.
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What's cool is that same vintage effect can be done with the MPA Gold.
Yeah... That´s one of the reasons I have this pre (impedance selector). The other two are instrument input (so can be used as a DI) and the analog output control so, if you whish, you can cranck up the input gain, obtaining more "colour", and not saturate the converter using the analog output pot.
In my opinion, versatility is the word that best describes this unit.