At some point the spacebar could be used to pause and play songs. I have no idea why this was removed, it seems like regression.
Please bring it back! 
It has not been removed.
But it is disabled in your preferences.
Just reenable the use of the media keys in the General page of Audirvana Plus preferences
Wow! Look at that. I assume this was an upgrade snafu or something else caused this!
Again, thanks.