I'm connecting an Apple AirPort Express to an s.m.s.l. Q5 pro audio amp. Both devices have analog connections and digital optical connections. Connecting via analog gives excellent music reproduction. Connecting via the Monoprice optical cable gives terrible music reproduction with about as much signal dropping out as getting through. I've tried two different cables with the same result.
The problem, of course, could be with either the Apple or the s.m.s.l. device (or both!). However, both optical cables looked perfect at one end and had a slight visible imperfection at their other end. I don't know what is to blame.
I gave up and am just using the analog connection, since it works well.
You have to give the product being reviewed a star rating; so I used three, the average of one and five. I hope my experience is of some value to someone other than me. Perhaps others have had related experiences.