The 550/570 blend is growing on me. I understand it to be what Ommegang uses to make Gnomegang. It isn't a 'classic' dubbel profile though if you are looking for competition points. I think blending the classic strains (500, 530, 540) with 570 in general for dark(er) belgian beers gives them a little more fresh fruit character, like pears, lemons and apples, which I enjoy. I have to believe those brighter fruit characters are present in the style when you drink them fresh at the abbey, but are lost in their travels across the pond. Dried fruit characters seem to hold up well to heat and time.