This is strange, the Icicle does not appear to have an output, yet you can assign it in AMS...?
Unfortunately I could not find a manual on Blue's website, just a shop leaflet PDF they call manual. Nowhere was any mention of an output though...
Does it have a headphone jack hidden somewhere? If so, that is where you should plug in your headphones or monitor system. If not, create an aggregate device in which the *Icicle is the Input* and your Macs *Built in Audio is the Output*. Otherwise you'll never hear anything from Logic.
http://support.apple.com/kb/HT1215
regards, Erik.