Mario: "Forget about General MIDI (GM) sound sources. The best sounds come from non-GM synths."
I do not fully agree. I find Halion and Sonic are quite capable GM synths at 20+GB of sounds, especially as "all in one solution" for the price point. All drumsets are adjustable-per-drum and many many other features. Both items come in one package and are usually 1/2 off at least once a year.
I was unaware how powerful Halion and Sonic are. With over 20 gigs of sound it probably is the best GM on the market. So I will retract my "Forget about General MIDI (GM) sound sources" and say "forget about most GM sound sources".
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Bottom line,
Kontakt by itself comes with pretty weak "factory" library. To get it right to "realistic specs", you have to hunt for sales/upgrades and by the end of the day, it will cost quite a lot, but you can achieve the "better" sound quality.
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I agree that the Kontakt factory library is very week and very dated. I also agree that the NI's sounds for Kontakt are over priced and that you have to wait for a sale to purchase them. But there are a plethora of excellent third party sounds ranging from free to very expensive. I have a large library of them that I have collected over the years. Kontakt is my go to sound source for most of my sounds.
Peace and stay safe.