I haven't tried the Devialet with ProAc yet, but seeing as the Devialet seems pretty much void of character (I mean that in the positive sense), they should work fine together. I haven't found any speakers yet to not sound good with the Devs. So with the fact that the Devialet won't change the sound of these speakers, we can just compare speakers really.
We have two quite different speakers here. The ProAcs will impart a certain amount of character to the performance, and it must be a positive thing as they're very popular. I've not been overly keen on speakers with ribbon tweeters unless they're accompanied by a dedicated mid driver, but the D20R and D30R seem to be an exception to that rule. Despite ProAc's character, I do find the D20R and D30R models a little more balanced than the standard models, and some of the previous models. How long have you had these? And did you compare them to anything when you bought them? What was your reason for choosing them?
The KEFs are about the same price as the ProAcs really, although a little more once suitable stands are used. Like most speakers there will be a certain amount of character, but you should find them a better speaker in technical terms if you take into account the UniQ driver. Speakers at this sort of level generally play all types of music well, but I feel Reference particularly excel with electronic music, and the ProAcs can really come into their own with real, non amplified instruments (string quartets, classical guitar etc). It'd be interesting to hear how ProAcs would sound when SAM'd, but unfortunately the only floorstanders they have made available for this (D40R) are now discontinued! Hopefully more ProAc models will be available soon.
Which one you prefer is really down to your preferences, and also if SAM is available for them, as this can change the end result by some margin. As it stands, both models aren't, but if SAM appears for the D30R, it would be advantage ProAc - the difference on our Blades was very surprising.