Treasure bottle / map quality

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Hello, 

I am running a small family blackwood server and I would like to increase the quality of maps respectively the bottles so that we can find maps higher than blue. And even those blue are still very rare currently. We collected much more than 100 maps and most of them are just not worth the effort. 

Please keep in mind that I don't want to have a general higher item quality or easier maps. Just higher quality and thus more difficult treasures for a better reward. 

It would be great if somebody knows a server setting for that. I can't find anything related, not even with Google.