The modifier in question is called "reduce-ac" and its value is to be interpreted as a percentage. In 1.07-1.09 the target defense is set to the value specified by "reduce-ac" while in 1.10a+ target defense is reduced by the value specified by "reduce-ac".
Arreat's in 1.07-1.08 has "reduce-ac" 15 while Eth in 1.07-1.09 has "reduce-ac" -25. In 1.10a this changed so Eth has "reduce-ac" 25.
That means for regulars/minions:
1) 1.07 Arreat's in 1.07-1.09 sets Defense to 15%.
2) 1.07 Arreat's in 1.10a+ reduces Defense by 15%
3) Eth in 1.07-1.09 sets Defense to -25%. (effectively giving Ignore Target Defense)
4) Eth in 1.10a+ reduces Defense by 25%.
I assume penalties against players/bosses/pets would still apply.
No clue on interactions with Stone Skin/Cold Armors.
1.07-1.08 Arreat's doesn't have Crushing Blow so it sucks anyway in 1.07-1.09.