Priorities...

Which would you prioritise for an upgrade - mono blocks, pre amp or speakers?

Depends on what I currently have.

whats your present system?

As a general answer to a general question and assuming you have all three tagged for an upgrade I would upgrade the preamp first and the amp situation after you have chosen the speakers you are going with. The preamp is I believe the week link in most systems and more than often over looked. Good luck

First, what the goals of such an upgrade? Without knowing those, the question is too broad and without sufficient information to answer meaningfully.

And what about the source(s)?