There are quite a few things which can affect the amount of crema produced, discounting your particular equipment setup and operator technique....
The freshness of the roast is definitely one and compared to the stale, old beans you get from supermarkets :o, there just isnt any comparison. Another variable is the particular blend of beans being used..... thered be a good chance that the Merlo blend you have has a percentage of Robusta beans in it and this will certainly increase the volume of crema, but not necessarily the quality :-?.
The type and manner of processing used for the beans being used also has an effect on the amount and density of the crema, i.e. in simple terms..... dry processing versus wet processing, or natural sun-drying versus industrial drying and the list goes on.
For the best quality coffee in the cup though, you can not beat roasting your own from high quality beans such as we have access to here at CS 8-) (lucky devils we are). You should consider trying it out for yourself, its very easy and inexpensive to get started and the results have no comparison, except with what other CSers are doing ;D. Once you start, therell be no turning back,
All the best,