Zone 6b is -5 to 0 degrees F, right? Very short list of plants that cold tolerant. I can't speak for the accuracy of the list in the link, but anything remotely approaching the limits of an agave in that range will require the plant be kept dry, pretty much. No experience with cold here, but we do have a thread from someone who had good results in a warmer (but still cold for agaves) climate. Maybe check it out.
The thread "Agave survival in the Pacific Northwest" in
Cactus and Succulents forum
