usb dac for hi-end system

I suspect it's because there's a larger market for USB gear, and a smaller market for firewire gear.

Ayre QB-9, end of story.

The Ayre is a very good USB dac but it is not the end of story. You need to listen to several in your system. The Ayre is optimized to run on USB and only offers a USB input. The technology Ayre utilizes is licensed from Wavelength which was a smart move on their part. There are several other options out there however that exceed the Ayre's performance but at it's price point it is a terrific value.

well actually ı have tried PS audio perfect wave dac it was ok but and also compared minjack toslink connected to scarlatti dac which was far more better despite that cable was a no name mass market toslink.
I will now lokk also to bel conto usbto spdif and weiss int 202.

I don't mean to get into the USB / FireWIre debate, but what about Asynchronous USB?
Doesn't that clear the board of some of the USB issues?
At a substantially higher cost?