Hi, everyone!
I just wanted to report my recent funny discovery of yet another USB audio device, perfectly working with iPad2 via CCK: Logitech Z-10 USB speakers. While not precisely the type of devices discussed in this thread, this is still perfectly eligible to be mentioned here, as what it really is is a USB DAC/Amp inside a speaker cabinet.
The device works fine via CCK, playing all sorts of tracks (including basic MP3s via iPad standard Music app, and all sorts of lossless FLACs and ALACs via Capriccio).
But more importantly, there's a very high level of compatibility of additional functions: its playback control sensor buttons (play/pause, Prev and Next) are fully functional and control the iPad playback in either of the two apps I tried. The iPad volume control controls the volume of Z-10, with respective indication on the speaker's display. Vice versa, the Z-10 sensor volume controls control iPad volume with respective indication. Mute button also works fine.
The only thing that does not work is the extended functions of the LCD display, such as track name and other tagged information.
I don't know what kind of DAC is used in the Z-10, but I can responsibly say that the sound quality is much better with digital connection via CCK, compared to analog line-out connection via Apple iPad2 original dock with line out (LOD).