Since acquiring one of Scan's brand new PC audio laptops six weeks ago (which uses the J Micron firewire chipset), and connecting that to my AudioFire8 soundcard, I have also had endless issues with crackles on playback, making the audio sound as if it comes from a vinyl record. It helped to increase the buffer size, but didn't eliminate the problem.
When I loaded Cubase 6 and used the AF8 asio driver the crackles were also there, and worse, when recording audio it sounded distorted and jittery, as if different bits of software were operating on different sample rates. Checking and changing settings in Windows 7 didn't help much, nor did changing the driver version of the AF8.
What did help was to load asio4all and set Cubase to that. Both playback and recording work perfectly now. Strangely this setting also improved non-asio playback (ie streaming Internet audio or playback via VLC or Audacity), despite the fact that W7 is set so that the Asio drivers bypass W7 completely.
Using asio4all is a solution of sorts, but it'd be nice if the AudioFire driver works seamlessly with W7 again. I'm in touch with Echo about this, and will post news when I get any.
Hope this helps,