This sludge backingtrack is written in the key of c# minor , the guitars are tuned low to high c# g# c# f# a# d# . On this track there is no bass. You can listen to the full version of the track as well, there is a link in the beginning of the video. The obvious choice for soloing would be using the c# minor pentatonic, you could use c# natural minor as well. Feel free to experiment with other scales as well.