Priorities...

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)?

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.

Although you guys may have other speaker recommendations...

Given you could afford it, would the GamuT amps be the ones you would want to keep for use with the DC10As? Otherwise, if you had to sacrifice the amps for new speakers, and potentially exchange to a less expensive/quality amp, you could be defeating the goals desired in your speaker upgrade. Also, two variables (amp and speaker change) could "cancel" out any intended "upgrade." Of course, if the amps are not being used in system then my points above don't matter.

Personally, I am a long time Vandersteen and Thiel fan.

Yes if I could afford it I would certainly keep the M250i monos...
I was initially considering used Rogue Apollo monos as a replacement...
I have already thought about what you say regarding defeating the speaker upgrade so wanted the opinion of others.

I realise the alternative is to not bother & just continue to tweak the system to completely optimise its performance.