What is a "SB" output?
You need a soecific speaker output for the 6th speaker. It's generally called something like "rear center" or "center surround" or something like that. That "SB" has me stumped.
You may have to tell your receiver that it's hooked up in one of the setup menus and, even hen, it will only be active wheh the source mandates for it. My Denon has an indicator light in the front panel that shown then a 6th channel is present, which isn't very often,