The answer to your question is that you can buy and install parts in the existing cabinets to (more than likely) get better sound than what you have now. To duplicate the sound of the Proac is probably not realistic. Unless you have them for comparison side by side. There are a number of sources online where folks who have practiced the art of speaker design for a long time share their designs for free to people looking to DIY stuff.
Everything linked below can be purchased from http://www.madisound.com
Troels Gravesen's CNO-25 uses very nice components (I can vouch for that, my speakers currently use the same drivers)
http://www.troelsgravesen.dk/CNO-2.5.htm
Zaph Audio's ZRT 2.5 way (scroll to the bottom of this page) has drivers that a lot of people feel are some of the best around. The air circ tweeter and the scan speak 8531 midbass drivers. I built this speaker for a friend of mine, he is thrilled with it.
http://zaphaudio.com/ZRT.html
This one may not meet your criteria because of the smaller woofers. Very, very nice drivers. Not sure on the crossover implementation as I have not heard this design:
http://www.madisoundspeakerstore.com/2.5-way-speaker-kits/seas-delling-excel-kit-pair/
There are lots of options. I've only touched on a few. It would take a little research to compare your enclosure size to ones needed for the builds I listed above. Just some food for thought.
Everything linked below can be purchased from http://www.madisound.com
Troels Gravesen's CNO-25 uses very nice components (I can vouch for that, my speakers currently use the same drivers)
http://www.troelsgravesen.dk/CNO-2.5.htm
Zaph Audio's ZRT 2.5 way (scroll to the bottom of this page) has drivers that a lot of people feel are some of the best around. The air circ tweeter and the scan speak 8531 midbass drivers. I built this speaker for a friend of mine, he is thrilled with it.
http://zaphaudio.com/ZRT.html
This one may not meet your criteria because of the smaller woofers. Very, very nice drivers. Not sure on the crossover implementation as I have not heard this design:
http://www.madisoundspeakerstore.com/2.5-way-speaker-kits/seas-delling-excel-kit-pair/
There are lots of options. I've only touched on a few. It would take a little research to compare your enclosure size to ones needed for the builds I listed above. Just some food for thought.