The Legends people at their sevice hotline tell me they use "premium" resins. I have no particular idea why they seem so hard to track out for most people. I just take them at their word, that they got the best they can afford to put in their balls.
One other detail that seems to help their balls be what they are: they use no fillers between the core and the coverstock. The coverstock material itself covers the entire core. FWIW my driller thinks their cores are very hard. he doesn't say that about many cores; he also thinks Visionary cores are harder than average.
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"...for advice is a dangerous gift, even from the wise to the wise...."
J. R. R. Tolkien