I used iTunes (x64 bit).
In the configuration settings for each OS, I made sure the OS (Leopard and Win7) was not doing any conversion by setting the audio output to 48kHz and playing WAV files ripped from CD.
Also, iTunes has the EQ on by default on Snow Leopard where iTunes on Win7 did not have the eq on, so I turned off the EQ on Snow Leopard before doing any listening tests.