Well the national has a high and low gain setting. On high gain it's one of the most powerful amps out there for it's size. My problem wasn't the power at all but the dark sound that comes out of it. Paired with the mad dog and you really need to like a dark sounding headphone. Dark to the point where it's veiled.
Actually for me, the darkness wasnt really a issue. I tend to lean much more towards dark sounding gear.
When I had The National on high gain setting I could hear it straining once I got to a 4 on the dial. It ran out of clean power and gave a ton of distortion at that point. The RxMKIII-B+ doesnt do anything close to that.