I'm looking for four discreet codes for the Pioneer VSX-D557. I don't have a remote for this unit at all so I don't even know if the IR codes exist, but the front of the stereo has two buttons that toggle speaker A & speaker B on & off.

I'm hoping it supports:
Speaker A On
Speaker A Off
Speaker B On
Speaker B Off

Pioneer doesn't change their codes between models.  Go to the Pioneer site, and choose one of the VSX receivers.  You'll find 5 codes related to speakers, iincluding discrete on for each of A and B. As I recall, discrete off affects both A and B, so there is no independent off for A or B.  

Thanks for the link, but unfortunately those codes don't seem to work with the VSX-D557. The Speaker A ON code is the only that the receiver responds to, and it actually is the Toggle On/Off for Speaker B.