[GreySkies]

Many of those simple converters only do 2.0 out, which could be why you're only getting Prologic. I've got a Pi3 that's doing 5.1, and another that's doing 7.1, but that's via HDMI. Here's a converter that claims to extract 5.1, including DD+.

Or, it could be that your settings have been set to 2.0, in which case you'd just need to go into your settings to tell the Pi3 to output 5.1.