Yeah, they're really different. How about SET vs. PP? The Sigs are also spectacular but mellower, smoother and more refined. The AMP-1 is ballsier with a bigger jump factor, more obviously dynamic.
Depending on the rest of the system and preferences, I could see either being preferable. I've made significant changes to my rig since I owned the AMP-1 but I have no problem with big dynamics so I don't need that function from the amp. I feel the Signatures are better for my system.
I still wish I had the AMP-1.