I owned Heresy's for many years. As Sean says, they'll play loud...that's the good news. The bad news is that they are mid-range forward and light on the bass. Your room is pretty big so you may not get below 60Hz. A sub is needed especially for rock. In a 20x25 room Dunlavy's should work well. I've heard the Cantata and Aletha and both are worth a listen and wouldn't need a sub (these speakers have great bass, actually the best I've heard).
Oh, by the way, listen to Kelly's advice on the Avant Garde's. My ears, too, are bleeding. Dave