I have successfully grown musa basjoo for the last five years in heavy clay soil. I simply dug in a lot of bagged manure and compost around a planting hole about 2x2x2feet, which I filled with commercial triple mix. Every year I add a few inches of municipal compost and bagged manure. The bed is slightly raised and being up against a south wall of the house is well drained. Musa B. is a surprisingly tough plant once it gets established.

I have grown hundreds of elephant ears, cannas, hibiscus and other tropicals in typically clay-like soil for over 25 years. I also have four outdoor palms and lots of banana, both musa and ensete. If you mix in a bale of peat, a couple bags of soil prep, maybe some steer manure, and sand (if you wish), you should be fine with the soil that you have. I continue to add whatever organic matter I can find to the existing beds each year. The soil just gets better year after year. I believe you can turn any clay soil into a good loamy soil with a little patience. You might add some milgorite fertilizer to the soil in addition to other common fertilizers. Good luck!