Is Spotify also available when Jooki is offline?

No. Unfortunately, at this stage, offline mode for Spotify is supported on smart phones, but not on Spotify Connect devices such as Jooki.

At this stage, if you want to use an offline mode you can upload your files (mp3, aac, ...) to Jooki. 

Is this likely to change in the future? We sincerely hope so. We realise that offline Spotify is an important use-case, being a portable speaker. However, this decision is under Spotify's control, not ours.