The volume control is provided by the hardware device, so I don't think I have any control over whether it is dithered or not. A quick check of Audirvana's code confirms that I'm doing essentially the same thing, neither of us are doing any dithering. Since I haven't tried this on a great many devices, I haven't come across the scenario of having no hardware volume control, but I like the idea of simply removing, or disabling, the control.
As for leaving volume control to iTunes, this isn't inline with having a high fidelity player. When you adjust volume in iTunes, it is a software adjustment. This works by changing the bits of the audio stream, compromising bit-perfect output. While it will probably still sound pretty good, it is always preferable to use hardware controls.
Due to the way BetterSound works, I don't think support for other sources of sound is feasible. BetterSound is really just a music file player which happens to be told which file to play by iTunes. Making this work for movies would be near impossible, unless I code a movie player too! Almost all movies have low bit-rate compressed sound, so it isn't really worth while. In principle it would be possible via browsers and such, in practice it isn't since the browser is generally not playing a file (it is streaming music) and it is not sending system wide notifications of what it is doing (sort of what I'm relying on with iTunes.)
The way PM does this is (I think - haven't really looked into it yet) is with a virtual sound device, kind of like Soundflower. This would allow the PM engine to get sound from any program and play it through PM, but there is a major problem with this. A great part of the improvement in sound quality comes from having one program, playing one audio stream having complete access to the output device - hog mode. As soon as you have to mix streams of sound from different programs the sound quality is affected, that might explain why SQ through PM's plug-ins is not great.
