About DR. I like the way that the BoVD handled it. High powered fiends were resistant to weapons of less than +6 or +7 enhancement, but the amount of reduction was only 15 or 20 points. I think (I borrowed the BoVD from a friend, don't have it with me) that the numbers were 15/+6 and 20/+7. The 10/+5 you use as a downgrade of the paragon's DR fits right in that progression.
Now if 20/+6 gets downgraded into a 10/+5 (paragon to phenix), what should I do with the Gloom (25/+6 DR)? Change it to 15/+5? Then the rule for DR of +6 would be to reduce it to +5, and lower the damage portion by 10.
As a general rule... maybe halve the amount of bonus over +5 (rounding down, so +6 becomes +5, and +7 or +8 becomes +6). That's equivalent to adding 5 and dividing by 2. How much to lower the damage reduction by?
An infernal has 35/+7. Say that goes to 15/+6 (the BoVD number). We've subtracted 20.
Maybe lower the DR by 10 if the final bonus is +5, 20 if the final bonus is +6, 30 if the final bonus is +7, and by (10X-40) if the final bonus is +X.
So a heroic hecatoncheires would go from 60/+12 to 20/+8. Too weak?