Jitter is often misunderstood, and many DAC's do not handle Jitter very well. Quite often, the high frequency jitter is filtered, but the low frequency jitter (or wander) is not. The Nuforce brand of DAC's do a good job of jitter reduction. Also, it is interesting to note, that playing a song on a CD generates very low jitter, whereas playing the same song through a SPDIF interface generates considerably more jitter.
My solution to jitter issues on my Maverick TubeMagic was to upgrade to Windows 8 and use an add on card to create a separate USB channel just for my DAC and ADC. Prior to that, all the telltale signs, afterwards ... perfect!
W8 made some major gains in audio handling with their improved WASAPI engine, rated to be "next best thing to bit perfect".
The add on card isolates the audio equipment from all the other USB devices we litter our computers with nowadays. Major issues with my Gyro AirMouse and wireless keyboard before the changes.
"Asynchronous" is touted as the end all of jitter reduction, but the way it's implemented is pretty much a crap shoot. Luck wit dat, eh.
** And yes, I do use the USB interface. Never had any real issues that could be attributed directly to the S/PDIF optical - I just prefer the sound quality of the USB, even though it tops out at a lower rate ... Once again, S/PDIF is a standard that's not always implemented well in hardware and tends to be more susceptible to bus interference issues.