I have BitPerfect (latest version .29, I think). A couple of things I've noticed, I don't think have been mentioned in this thread:
First of all I'm using it with a Mac Mini, with USB out to a Wyred4Sound DAC-2, then on to a Violectric V200 and then to my LCD-2s. I've been trying different settings with BitPerfect's preferences, and it always seems to work fine, and offer an audible improvement, except when switching tracks with different sample rates on the fly. If I try to do that, there is a tiny glitch, while the new track stutters; it's just a half of a second tick; then the track plays fine. I've noticed no other freezes, track skips, etc., that were reported with this and some earlier versions.
One thing I've discovered about iTunes is that the newest versions (10.4.0, 10.4.1) have different settings available in Audio Midi setup, depending on which kernel the Mac is booted into. For example, I have another iMac, which has the Intel Core 2 Duo processor, so it's capable of being booted into the 64 bit kernel. I do not have it set up to boot that way, however. In Audio MIdi setup, choosing the built-in output, my sample rate choices are 44.1, 48, or 96, and my bit depth choices are 16, 20, or 24.
On my Mac MIni, however, I have it set to boot into the 64 bit kernel. With the same version of iTunes, (10.4.1) the choices in Audio Midi setup are 44.1, 48, 96, and 192. Bit depth is fixed at 32.
When trying to determine if BitPerfect was improving the sound at all, I did the following; I first bypassed BitPerfect entirely, and in Audio Midi setup, I chose the DAC-2 as the USB output device, and set it to 192K, and the default (only choice) 32bit/2 channel. This way, I wouldn't have to manually change sample rates for different tracks; the DAC-2 would simply upsample everything that came its way. Things sounded pretty good with this setup.
However, when putting BitPerfect back into the chain, (with upsampling turned OFF in BitPerfect's prefs), the sound was noticeably better. More solid bass, more transparency, and even slightly louder volume, without changing anything else. Now the DAC-2 is getting the native sample rates of all the tracks, changing them when necessary. Audio-MIdi setup doesn't matter any more, since BitPerfect is 'hogging' the device. (Of course, the DAC-2 must be chosen as the output device in Audio-Midi setup).
As for the other BitPerfect prefs, I have Integer Mode turned off (since the DAC-2 doesn't support it, anyway) and I have the 'Output in Max Bit Depth' and the 'Use Maximum Device Buffer Size' boxes both checked, although it doesn't seem to make any different in the sound whether they're checked or not.
The tiny problem of an audible tick when switching tracks on the fly is minor; I only do that to test it, anyway. Normally, I would pause the track I'm listening to, choose the new one, and hit play. Then, if the new track has a different sample rate, there is no glitch; it starts and plays smoothly.
So all in all, these settings are working great; with BitPerfect in the chain, there is a definite audible improvement.
One last item about iTunes; I was pretty sure older versions could only accept up to 24/96 tracks, but I've downloaded some 192/24 flac tracks from HDTracks, converted them to AIFF using Max, and when doing a Get Info on them in iTunes, the 24/192 shows up. I haven't tried this with the 32-bit kernel iMac, though.
I hope all this might be helpful to anyone using similar gear.
