Proac 2.5 Response tweeter upgrade with Ribbon Tweeter?

Glad you like the Tannoy speakers. I am sure they are a much better match for your amplifiers.

 "The sound stage is huge and 3D (compared to my Proac 2.5 and the D48r's) "

If you don't have a huge 3d soundstage with the D48r something else is wrong

(((If you don't have a huge 3d soundstage with the D48r something else is wrong))Yes in total agreement. They play music without ringing artifacts.To some folks, ringing offers a short term wow factor.
 Best JohnnyR

2. If I want a speaker of this ’ilk’ (2 woofers + Tweeter and crossovers) that needs serious drive power then I would need to purchase a new amplifier. However, I would rather investigate other speaker options so that I do not need to change my amplifier.

As a general rule of thumb, if you want to get the most out of your tube amplifier, its in your best interest to avoid 4 ohm speakers- and this includes speakers that are rated at 8 ohms but have dual woofers such that the woofer impedance is 4 ohms.

So when I see dual woofers I make sure to find out what the impedance is at the woofer frequencies! If its 4 ohms its going to put your tube amp (regardless of the tube amp) at a disadvantage.
Now its also a simple fact that most amplifiers regardless of technology make less distortion into higher impedances. So 4 ohms means in general you will be hearing more distortion- and the ear converts all distortion into tonality, so this usually means 'brighter and harsher' with less detail than the recording itself. All in all its worth it to seek out those speakers with higher impedances- for one thing it means that the designer is listening to his product with a more discerning ear.

Thank you !
8 Ohm's it is. I've heard the results of 4Ohms first hand now - not pleasant.
Thx, Bob

I have exactly the same speaker and was looking to do exactly the same thing. 
It would require a lot of cutting amd fiddling about to put a ribbon and won't be cost effective.
The best option would be to buy the d30rs dome tweeter from proac directly. Ill be looking to do this after the new year so ill let you know the reolacement tweeter price