Q: No preview sound on Apple Loops

Hi there, new to this, new to Mac, new to Logic, so probably many questions to come.
We're gradually working out how to record via a Motu interface. After much shenanigans we've finally got the ins and outs sorted so we can hear the instrument we're playing, and the resulting recording, through the headphones plugged into the interface.
Trouble is, now we're getting no sound when we play a loop preview in the loops window. We can hear it if we drag it onto a track, but you need to be able to hear the preview in order to choose which loop to use.

Any help would be greatly appreciated.
Cheers