So an easy way imo to fix Daemonhosts(and by extension other monstrous creatures/assassination targets) is to add another currency. "Incense of the machine".

By beating a boss on Malice you can get a incense which could boost any stat by up to 9% of its current. Each Incense increases a modifier by 1%. Once three stats have been adjusted this way you can only use it for 5% on the remaining stats. Once capped in this way the items stats get an green lock next to them.

By beating a boss on heresy you can further increase the buffs by another 6% if they are the main three stats you picked before. And 3% for the remaining. Once capped the lock will change to amber.

By beating bosses in damnation you can increase the cap one again by 5% and in the remaining stats 2%. The weapon stats will now show a red lock to indicate you cannot raise them further.

The items dropped would be ranked as

Mundane incense of the machine (white) Superior incense of the machine (blue) Transcendent incense of the machine (orange).

Edit:

I had this discussion in the forum too but:

A successful clear of the Daemonhost encounter results in Not killing the host, killing the host would result in no reward.

Thanks for those that read the post and had constructive feedback that hit the main point of the post, which was a way of increasing boss rewards by providing one, and providing a way to increase modifiers.