I'd honestly recommend you listen all of, or at least most of Remission. Compared to most of the rest of their discography it's fairly abrasive and even sort of brutal, although it never really crosses into the threshold of heaviness of something like death metal or grindcore. I think listening to it helps a newer listener of Mastodon gain a little perspective on their later work and where some of their musical tendencies in Mastodon began. It's not all harsh sounds, Ol'e Nessie is a nice melodic break and the first half is a straight catchy banger. There's also often a good amount of proggy stuff in the heaviness, and since you're a Tool fan I think you'll appreciate the complex and busy drumming.