US 366 main volume control not working (iMac - yosemite)

[Ramps]

Hello, I'm using a US-366 on an iMac (OS X Yosemite). Up to now I've had no problems as I've used nothing but the headphones output; however, I've just connected a pair of yamaha HS5 speakers to outputs 3 and 4 only to discover a slight glitch. I've negotiated most of the small problems of the set-up but I can not figure out why the main (big centre dial) line out volume control does not work. I have to control the volume via the volume controls on the back of the speakers. Any ideas? Everything else is running fine but this is a bit inconvenient and I'm sure there's a solution.

Thanks in advance.

 

[Holm]

Owner's manual says:
LINE OUT knob adjusts the output level of the line output jacks (OUT 1/L and OUT 2/R)

 

[Ramps]

Thanks Holm. I'll get a different set of leads and try that.