how do you mean harsh? acidic/sour? you could try adding chalk or only adding dark grains in the sparge. or cold steeping them. have you measured the pH of your dark beer mashes?
Use less have you made a 80/ before? can be dark but is always smooth and lovely.
For porters you don't have to overdo the dark malts, choc shouldn't be too harsh or you used to much, match it up with some munich malt for malt depth and you have it.
I can see a heap of brewers arking up saying add the dark grains to the sparge but i think if you get the recipe and everything right to start with there no need for that.
I agree with Jayse' comments, also what type of dark malts are using. Roasted barley can be a big contributer to harsh flavors if you use too much as can Black malt.
I can't comment on AG variables, however I have done two 2 can recipes that were nearly identical in recipe but one was harsh in flavour.
Both brews involved 2 cans of goo (stout+pale ale) with 150g of choc grain and 100g of dark grain.
The only differences was that smooth stout used the yeast that came with the Country Brewer kits + licorice whilst the harsh brew used White Labs Irish Ale liquid yeast (and no licorice).
The harsh beer fermented at a higher temp so I assume this is what caused the harsh flavours.
The moral of the story is not to alter two variables at once!