All in the set up?
 

Just got my double bass back from an international transit and the E string sounds really dead with absolutely no ring on pizzicato - just a dull thud. Arco its ok but just sounds muddy.

Looks like the sound post may have shifted such that it is directly under the bridge foot rather than offset towards the tailpiece direction. Could that be causing it?

I had a new sound post fitted and its a 15mm rather than 19mm could that have something to do with it? ...could the sound post be too long and stopping the top from vibrating properly as the lowest frequencies ( it seems to be a very tight fit)

...just thinking that maybe the change in temperatures in these winter times + travel means that I may need to get the sound post adjusted and trimmed a slice or two.

The bridge seems centred of the f hole nicks but I have heard of people saying that the slightest shift in the bridge can affect volume and tone. All I know is that my E is dead! Anyone else had this problem and found the solution ( I am going to get the sound post adjusted in any event as clearly thats wrong anyways! ).

Take it to your Luthier and let him figure out what happened. If you bridge was bumped, the string itself may have been damaged. If the soundpost moved, it would be because it is too short and not too long.