The Proacs I've heard have been clear and dynamic, somewhat more analytic than my taste (and than the Time Windows), but attractive nonetheless. But I suspect they are not particularly efficient, so if the tube equipment is not high power, you might not get the full benefit of that dynamism. I'd listen carefully before deciding, since, in my opinion, a pair of Time Windows running on tubes in a small room is a pretty good combination, and not so easy to beat.