Cartridge Impedance

Changing the impedance from 100 to 200 ohms with my Denon moving coil increased the clarity and focus and the highs sound better. 

The cartridge impedance is what it is, there’s no such thing as adjusting the impedance of the cartridge. What’s adjusted is the impedance or load the cartridge sees. Top end changes are easily noticeable, but there’s more to it than that. Reading up will help, but you learn a lot more by just doing it. https://www.mysonic.space/phono-cartridge-impedance-loading

1) With higher load impedance on your phono stage you're actually unload the cartridge, you have to try this if you're looking for more open sound and better high frequency. You can try anything up to 47k Ohm for MC cartridge. Don't be afraid to try 'electromechanical damping' as it too can be tuned and you may like it also, even possibly better.

2) With lower load inpedance on your phono stage, rather than unloading by going high, this in fact causes the coil to produce more current and hence this introduces electromechanical damping - the output will be lower, so keep that in mind, and explore the final value, deciding by listening. Personally i've find a range of 100 Ohm - 2200 Ohm very pleasant for 99% of my LOMC cartridges. However, there are some cartridges that can be just fine with load impdance lower than 100 Ohm. 

Anyway, it depends on the personal preferences and recomendations from the manufacturer. They are normally declare anything higher than 100 Ohm and you can't make harm by going anywhere up to 47 000 Ohm. If your cartridge is not bright then going higher (unloading a cartridge) is better in my experience. Your experience can be different in your system. 

Thanks to all! You've answered my question.

You're welcome @jmh128 :)