from all my research it was the Sierra II that had 6 tubes, not the Sierra I
Sierra I had 8 tubes, 4x65cm, 4x53cm. It had the Victor III bow sonar.
Sierra II had 6x53cm due to the enlarged bow sonar from the Akula taking up too much space.
They also had room for 40 weapons in the torpedo room.
Here is a decent article as an example:
http://www.hisutton.com/Russian%20SI...ack%20sub.html