Am I too late to make a comment? I use mushroom compost on my allotments. It gets dumped on the raised beds in autumn and dug in in the spring. It really improves soils structure but can't be used on its own.
Mushroom compost has become very popular and therefore extremely expensive, as it has taken over from horse manure. People still don't trust horse manure due to the risks associated with herbicides used on the crop that makes up the hay that feeds the horse. There is a huge pile of horse manure on my neighbour's allotment. It has been there for three years and only now there are a couple of distorted broadleaf weeds growing on it.
Strangely, no such problems have been reported for mushroom compost. This is surprising, since one of the ingredients is horse manure. Maybe there is more security of the source of the herbage and how it has been grown ie. which herbicides, if any, have been applied.
To come back to the main topic, I have found that ordinary potting compost (I get mine from Wickes, it's cheap), mixed with grit (2:1) is perfecty good for a raised bulb bed.
