There have been several acts that have stood as the totems of instrumental rock music, or post-rock in the broadest sense of the word: Mogwai, Explosions in the Sky, Godspeed! You Black Emperor and Sigur Rós all acting as cornerstones of the genre’s respective corners. For the past 26 years however, Chicago five-piece Tortoise have been the oft-unsung heralds of a scene’s progressive evolution, gradually bubbling under as one of those defining acts, cementing their influence on the countless bands that have followed a similar stylistic path. Originating as a “rhythm section for hire” of sorts, the focus on rhythm…