Hi Russ,
this is normal. The LINE IN's are designet for DJ Gear with high Level Out's like CDJ. CDJ's got nearly 2 Volt at the RCA. Normal Consumer Line Level is 0,3 Volt (Home Hifi / Smart Phones ) !! Your PA AMP is designet for 0,75 Volt / 0 dBu . (in some cases switchable to 1,4 Volt).
So if you like to use your iPhone --> use the USB PORT and your Phone with Rekordbox APP is like a USB Stick with full GAIN Level