Hi I spoke with Tascam support. They suggested for Tascam US-1641 to plug into a powered USB hub instead of directly into the USB port of the new MacBookPro 10.9.1. WOO HOO. sure enough fixed all my audio problems. no digital “jitter” and cubase A1 4 runs perfectly on the new OS. Using Bit Accurate as my Audio Source (Not core audio) Hope this helps folks. Thanks
CZ