The most important number when trying to balance bitterness in a beer is the ratio of international bittering units to starting gravity. This is often expressed as BU:GU (bittering units to gravity units). For reference, this posting has a more detailed explanation and some example BU:GU numbers for popular styles. Some Googling will get you some BU:GU numbers for imperial stout.
An imperial stout would typically have quite a high BU:GU -- close to 1.0. If that's not to your liking, adjust your hop additions to lower that ratio. Be careful not to bring it down too low, you'll end up with a sickly sweet beer. The bitterness is important to balance the sweetness from the residual sugars present in a big beer like RIS.
I wouldn't suggest changing the quantity of specialty grain to achieve a sweeter beer. Find a proven recipe and leave the grain bill as is and adjust the hops to achieve the desired BU:GU numbers.
The yeast won't have much affect on perceived bitterness. A higher attenuating yeast will produce a drier beer, which accentuates bitterness but this affect is small.
Longer aging, oak, vanilla, etc. have no effect on bitterness.
Sulphate accentuates bitterness. If your water is high in sulphate, or you're adjusting your water to increase sulphate, remember that this will increase the perceived bitterness.