WaveLab has perfect downsampling routines. There isn't much options. You can do it under batch processing to get highest speed. There you select the downsampler (set it oh High Quality of course), you select 16-bit WAV output format, the output directory and then you run it. Note than WaveLab will downsample in 32-bit floating point mode, than dither to 16-bit. So don't worry about that. Speed: to downsample a 20-min 24-bit Mono WAV @48KHz to 16-bit WAV @44,1KHz, it takes only one minute on my P3@667MHz.