Latest versions of iTunes/Quicktime here and no crackles. iTunes is on Laptop with Win XP Pro SP3. Connected to Beresford with USB. Never had any crackling issues with the Fubar II USB DAC either.
Start up Quicktime. 'Edit', 'Preferences', 'Quicktime preferences', 'Audio' tab, 'Sound out' panel... set 'rate' to 44.1khz and size to 24 bits. (Yes, 24 not 16)
These settings remain permanent.
Bring up 'Sounds & Audio devices' panel in Windows.
Click on 'sounds' tab.
Under 'sound scheme' select 'no sounds' now click on 'audio' tab and select your USB device under 'Sound playback' and click 'advanced' then in new window click on 'performance' tab.
Ensure Sample rate conversion quality is set to maximum and click ok. Click 'ok' on Sounds and Audio...etc box.
Bring up iTunes. 'Edit', 'preferences' and playback. Un-check effects and crossfade and Sound enhancer. You don't want them.
Always ensure the computer's system master volume (on windows) is set to maximum ('unity gain' is the posh term) and ensure the iTunes volume is set to maximum.
You don't want anything but your hifi controlling the volume. Digital volume uses up part of that 16 or 24 bits. Leaving it on max means the computer's/iTunes' volume controls are not deteriorating the signal.
(Don't use any USB cable longer than 5 metres and don't run it through any 'hubs'.)