What are some good dark malts for adding depth and richness without adding bitterness and burnt flavors?

I’m planning on making a dark ale, and intend to go with Maris Otter and flaked barley for the main part of the mash, and East Kent Goldings for the hops. I’ll probably use an English ale yeast.

I’m not much of a porter or stout fan, as the bitterness doesn’t really work for me. However, I’ve had a few dark beers that I like, including New Belgium’s 1554, Holy Mountain’s Black Beer, and a few dark lagers. What are some good ways to get the rich, complex qualities of darker malts, but also avoid the harsh bitterness?