Proac 2.5 Response tweeter upgrade with Ribbon Tweeter?

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.

It looks like you are using a tube amp with medium to low power. The Proac 2.5 is very inefficient, but can produce deep bass (see the review in Stereophile).

FWIW we sold our M-60 amplifier to a lot of Response 2.5 owners back when the speaker was in production. You don't need a solid state amp to get good bass on that speaker- you just need an amp with bandwidth (the M-60 is full power to 2 Hz). A solid state amp is not likely to get a 'sweeter sound' that is also sought.