I live in zone 5b, and have just tried Musa Basjoo for the first winter. This has been what I consider an unprecedented success. However I've made some observations.

1) All separated pups died even with protection. All small plants died even with protection. These were up to 3 feet tall at the point winter came. I think the key is to make sure that the plant has tremendous girth when winter comes. I got this effect by purchasing a plant which was already 3-4' tall last spring.

2) a wide mulch "footprint" is better than a "deep and narrow" mulch "footprint". When winter came I cut down my clump (a single main plant 13 feet tall and bunch of pups up to 3 feet tall). I covered this with some of the leaves from the plants, and then about a 5-6' wide pile of straw about 12" deep. This protected the ground quite nicely. I removed this pile in early april, and growth soon resumed.

Anyway, now my basjoos are the biggest in town. Everyone elses are about ankle high or so, and mine have leaf tips almost up to my shoulders. Interestling, the main stump (of last year's 13 footer) died, but the biggest pups have telescoped from their bases, growing into strong, healthy sword suckers. I didn't mess with cages or plastic wrap or insulation batting or trash cans or anything- just a nice, low, broad haystack, and I tell you what, I couldn't be happier with what I've gotten. Hope this helps for you if you try it; and I hope it works next year for me and wasn't just a fluke!