snip
I'd be pleased to hear anyone else's experience as I can't see how the
firmness of the soil relates to the firmness of the sprouts.

We always do the famous gardeners' dance on the back of the heels to firm
the ground down for all sprouts and cabbages that need to hearten up. Best
way I find is to clear the previous crop and just hoe the ground rather than
digging it and it is already nice and firm. Why it works is a mystery to me
but , on the whole, it does work. Like the others said though, any greens
that they produce are good especially the tops which are a late winter
treat,not to be missed.