It should be a +15% with pluto's.
With the higher Dark Magic skill now,
Diabolos's pole might be good to use for drain/aspir if you aren't getting resist. It adds +25% to drain/aspir I think and works under Voidstorm, which gives its own bonus.
Although I'm not sure how the new ToM staves work with drain/aspir. Haven't heard of anyone testing it out yet.