Subs should drop common crafting stones very commonly maybe 1/4. Mobs need to be dropping these too, but obviously at a semi rare rate. That way you can farm and craft gems with medium effort put in. I actually agree that sub bosses were dropping way too good, you could farm subs on rotation every 30minutes and get so much more value than if you simply camped euro bosses. Source: Every sin/tao in our guild camping them, from bene's alone you make a profit. The amount of 40 weps/44+ books coming from these SUBS was astounding.
Secondly Dan, some sub bosses need to be made better and some need to be made worse. Literally EVERY sub was the same dropfile on the 1drop system. The sub that's on floor 4 of CrystalCave for example that's a ballache to get to? Exact same drop as a frozenrobot that took 10seconds to get to. The subs that require effort to kill/get to, should be better hands down. That's all top tier cave subs.
Actual euro bosses need to drop RareCraftingStones way, way more often and drops imo need a slight buff. IWT's shouldn't be the primary source for this or mining takes over the game. Mid to High tier BOOKS should be coming from these 2hr+ r/s timer euro bosses primarily, top tier bosses with a slightly better chance obviously. I cannot remember the last time I saw a boss drop a book of value like FF/SS/Rage etc.
The "Big Bosses" with 6hr r/s timers I guess you could say the top 9 - EM/Apoc/FK/Swamp/Yims/OKS/MirKing/CrystalKing/Senjin, all imo need to be dropping a dc/mc/sc gem 1/1 or 1/2 with a RareCraftingStone and a rare chance for an EpicCraftingStone for the amount of effort required to GET to it, take it down and/or fight for it. At least this way you'd reward the effort somewhat even if you don't get anything of value.
Goodluck.