There are several ways to get terrible espresso with the La Pavoni single basket.
In the first place if one fail to use a large enough dose the tamper will hit against the sloped side before the dose is tamped at all or before it is tamped toward the centre. Make sure that there is enough untamped coffee so that when it is placed in the basket it is level across the top. This takes about 8 grams. Otherwise tamp as usual.
The normal double basket output from a La Pavoni is between 35 and 40 grams when the machine is properly bled, and when the group is at the correct temperature. You thus need to pull a shot with about 17-18 mL (or grams, espresso is near unit density). The easiest way to do this is to use a glass espresso cup and put 18 grams (mL) of water into it before ever making single shots. One can take a piece of tape and put it on the outside of the glass to mark the target amount. If you always use the same cup you will quickly learn the proper depth without the tape after making singles a while. You can do this with other cups, but you will need to use greater care. Too long a shot will be bitter as usual.
If you have a bottomless portafilter, use it for at least the first few shots. Especially watch the way that the first espresso appears when pulling the handle down. Watch it from a low enough vantage point to see the screen as the pull starts. You want to be sure you are getting an even tamp and distribution.
Watch the level of espresso in the glass cup as you pull it. When it gets to the tape, release the handle, remove the cup and put a waste cup under the portafilter and empty the rest of the group water into the waste container.