XA1 is a AudioUnit plug-in for Mac OS X, unlike the wast majority of analyzers that use FFT for analyzing the audio, XA1 use bandpass filters as most of the hardware devices.
Therefore XA1 has a very musical display, the algorithm used for analyzing has the same resolution across the spectrum, so the responce in the lo end of the spectrum is just as fast as the high end responce unlike FFT based analyzers that has very bad timing in the lo end.