Priorities...

Buy nothing until you know it will improve...bring delight to your music.   I would listen to your current system .....go to concerts, enjoy life.

Thank you all for the input.

I will mention again that there is nothing wrong with the sound of my current system; it's very enjoyable. The situation is primarily capitalising on the opportunity of a very good deal but requires a possible compromise. 
I say possible as I may be able to replace the GamuTs with a similar performing alternative. But it raises the question is the DC10A a quantum leap over the DC10T...??
This is where DC10A owners contribute with their opinions of their performance...

@carmenc - are you suggesting the M250i monos are not quality amps...?!

Not at all....just that you have lots of money tied up in all that "big iron" that is unnecessary for Tannoys.  $$$ that could be used to buy a nice quality integrated, which Tannoys do very well with.  For tubes think Manley Stingray, VAC, PrimaLuna HP,  Blue Circle ( what I use, but hybrid ).  SS think Pass, McIntosh, Accuphase, etc.. May need to buy used....that's ok.  That's a key concern, are the 10A's that much better than 10t?  Only you can answer that.

Username "sense63" has had experience with the 10A.  I believe he used a Primaluna HP to drive them.  Try to touch base with him.  That's the neat thing about Tannoys, you don't need to spend loads on an amp.  This is almost a 180 from all of my experience with Thiels.  Good luck.  Regards.....

@carmenc - the GamuTs are not unnecessary for the Tannoys; their quality is such that they reveal the full capabilities of the Tannoys.
What music do you listen to?

@infection what are your room dimensions?  It appears to me you've received some good advice from others here yet you're still spinning your wheels so to speak.  The Gamuts are unnecessary if you're trying to "steal from Peter to pay Paul" to afford the 10A.  If money is not an object then keep them!  The 10Ts do not NEED monoblock power.  Wanting that power is another issue.  Another way to go is with a tube preamp.  Many options....time for you to determine what's most important to you.

I listen to jazz, classical, fusion, vocals, Spanish guitarists, blues, international music and classic rock.  If you're playing rock and/or orchestral music in a large room, then I can understand your desire for power to control the bass on the DC10t.  Do you really like Tannoy speakers?  Have you listened to others?

Like others have advised, I wouldn't do anything or spend another cent until you've done more research, both outwardly and inwardly.  Good luck.  Regards....