Re: Musa velutina hardiness.
I think Musa velutina can be tricky to bring through the winter. You can luck out and plant them in the right spot and have them be very dependable for you, or plant them in a less-than-optimum spot and they will die over winter. Drainage seems to be key, but I think the more important factor is freeze duration. Here in zone 7a Tennessee, we only see a couple of days with a high below freezing, on average, per winter. A zone 7 further up the Atlantic coastline, like in Washington D.C or Maryland, can stay below freezing for several days at a time (I'm not sure about the averages for that area). If you are in a zone where you get more than say four or five days with a high below freezing each winter, then you should probably site M. velutina up against the south foundation of a house in a raised bed with excellent drainage, and apply a few inches of mulch for protection in winter.
This year was the first time I've brought it throught the winter successfully in the ground. It was only the second time that I've left it in the ground though, and last year's corm looked solid but with no growing points. I think termites ate on it. The other couple of times I've tried to overwinter M. velutina was under the house, and both times the corms turned to dust.