Hm, well, I have freshly roasted coffee now, from a reputable supplier and it is still tasting almost undrinkably bitter. My ratios seem to be right: 15g of beans yielding about 22.5g of liquid coffee in around 25 seconds.
There doesn't seem to be channeling, either: I'm using a naked portafilter and it produces an impressively unified stream.
I have only been locking in the portafilter a short time before preinfusing, though, because I had read that the portafilter could get too hot. Should I be leaving it in to warm up for longer?
I am feeling a little dispirited (not to mention broke) by my coffee experiences!