Hiss is weird. I don't know what could cause that. Normally digital problems will cause static like clicks or brief dropouts. Maybe the Ayre is interpolating over errors and the artifacts of the interpolation is being heard as hiss? I'm really grasping trying to guess what may be going on.
I'll brainstorm some troubleshooting steps and ideas:
176.4 kHz and 192 kHz requires the custom Ayre driver. Verify that the DAC is connecting with the Ayre driver and hasn't defaulted to using the stock USB 1.1 audio driver. The
Ayre usb setup pages explain how to check the driver. Depending on the order that devices get recognized during boot it could be possible that you're defaulting to the USB 1.1 audio driver rather than the Ayre driver.
Try different USB ports. Make sure you aren't using a USB port that is shared with other devices on the same USB bus. USB ports on the front of a computer may be all connected on the same bus so try ports on the back of the computer as well. When you change ports verify that you are reconnecting with the Ayre driver.
If using a desktop computer try installing a USB card and dedicate that card to only doing USB audio for your Ayre. USB cards are inexpensive. Using a dedicated card for only usb audio means the Ayre won't be competing with other usb devices on the same bus.
Try a different USB cable that is USB 2.0 certified. Maybe your cable is causing errors or problems at the higher sample rates?
Run the
DPC Latency Checker to see if you have latency problems. I'm thinking it is unlikely you have DPC latency problems because you're not having audio problems at lower sample rates like 96 kHz. Worth a try to check though since running the latency checker program is easy.