DD, im probably not the person to ask as im an asparagus newbie myself. I do know that they require a good well manured trench to sit them in though.
I dug my trench ,put a good dose of manure and compost in, mixed it up, left it a bit then planted my crowns and covered with compost and soil.
As you said a good amount of well rotted manure, and compost and the bed needs to be weed free, like as weed free as physically possible.
I always sieve the stones out too. I have found that too soft a ground like sandy soil isn't the best, the best soil that I have found is non stoney, well dr