Can't attenuate noise (PPC Mac)

Since my recent experience testing Surge on my PPC Mac was such a mixed experience -- great great synth, bad bad bugs -- I decided today to give the synth a spin on my Intel Mac at work. The difference between Surge's behavior on that machine and my PPC Mac at home was dramatic. All of the filters are stable on Intel. The Chorus unit performs as expected on Intel. I was getting some great Karplus-Strong type sounds really easily using the feedback configurations. So I thought I'd try that a bit here at home, hoping that none of the components I needed were broken on PPC. Instead, I ran into a really basic problem (again, PPC only, everything worked as expected on the Intel machine at work):

Start with the Init Saw preset. In the Mixer panel, mute all the other sources and unmute noise ("N"). Drag the Noise Level slider all the way down -- the tooltip value display should read -inf dB. In this state, I expect to hear no sound (noise is fully attenuated and all the other sources are muted). That's what I get on the Intel machine, but here on the PPC machine, I get noise loud and clear -- it's only about 2.5 dB lower than when the Noise Level slider is all the way up.