Priorities...

Ok I suppose I should offer a slightly more detailed explanation - I have the opportunity to upgrade my Tannoy DC10T to the DC10A for an excellent price but in order to fund this it requires the sale of my GamuT M250i mono blocks. 
Obviously this will leave me 'ampless' so I've been looking at decent used cheaper alternatives.

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)?

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

whats your present system?

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

Depends on what I currently have.