Sorry, but according to the description of drain soul, the damage should be increased x 4 when the target is below 25% health. Until we agree.
The problem is that when you start to channel on a target that is 18% say the damage is not multiplied by 4, if that adds about 500 damage regardless of the sp you have.
That tells me that the spell is bug, because it does not multiply by 4, just apply a flat increase.
A warlock with 400 sp gives the same bonus to one in 3000 with the sp. sum always the same, never multiplies the damage.
Have you tried the spell and you would multiply x 4? Greetings.