A quick tutorial on Volume Control in BitPerfect. There are two available volume controls, (i) the iTunes Volume control slider, and (ii) the BitPerfect volume control slider. When BitPerfect is playing, moving the iTunes volume slider results in digital volume control being applied in software by BitPerfect using its own 64-bit audio engine. By contrast, BitPerfect's volume control is a communication with the DAC, and asks the DAC to apply the requested amount of digital volume control. This only works if your DAC supports digital volume control, and not all DACs do. Also, some DACs report that they support digital volume control when in fact they do not, in which case BitPerfect's volume control slider will move, but nothing will change. Be aware that the generic protocol for digital volume control within a USB DAC provides for both attenuation and gain, although the DAC may only offer limited support.
The keyboard volume controls operate upon the BitPerfect volume slider. However, due to a known bug, these don't do anything if BitPerfect's volume control slider is visible. The volume control on the iPad Remote App operates, naturally enough, on the iTunes volume slider.
It usually the case that if you adjust the volume using the iTunes volume control while BitPerfect is playing, you can briefly hear snippets of the music playing through the Mac's speakers, or through some other device. This is entirely normal, and is not a bug per se.
If you are taking advantage of BitPerfect's DSD support and playing native DSD content to a DSD-compatible DAC, be aware that the iTunes volume slider will not function. The BitPerfect volume control slider may still work, depending on whether your DSD-compatible DAC supports it.