DDJ SB/RB users please help

Clearly not having (enough) power on the SB will hamper it’s proper working. It’s USB only powered.

A 5 meter USB extension cable is no problem. What you need to do, imho, is get a small, POWERED, USB-hub. Plug your SB into the powered hub with a regular cable, then run the long cable/extension to your iMac. Your SB should now have enough power and I am gonna wager a guess that your sound problems will be gone too. With regards to that, this is assuming you are not running very long unbalanced RCA leads to your mixer.