Silvias single basket is such a tempermental apparatus I really try to avoid it. It is so unforgiving when it comes to over-filling. I find that if its filled a little over the brim and then tamped -- as you would the double-shot basket-- the portafilter will not lock on the grouphead. *Underfill to compensae and you have a watery brew with very blond crema.
By comparison, the double is very, very giving, and can take much, much overfilling before Silvia decides enough is enough.
Packing in as much coffee as possible is a consistent way to get good, viscous, oily espresso with the tell-tale dark crema. BBut the single basket wont allow it.