Tube USB Preamp

I own the Cosecant, it is a non-upsampling USB Zero Dac, with a Tubed output stage. And like Paradales said there is a version that can act as a Premp with Volume. Actually Wavelength now has 3 different USB Dacs in the line-up, with varying versions.

There are others like the Bel Canto, Musical Fidelity, Benchmark, Hag to name a few. However other than the interface they are completely different in their design/philosophy;`using primarily commodity based DAC chips, upsampling, filters. The Wavelength DACs are a custom made product from the ground up, USB controller custom designed, power supply as well and so on. The sound I have experienced from Cosecant is mind boggling, from Brick sound is superb for the money.

From a design and research standpoint these products are simply incredible and have no equal at the moment. Here is the link to the site...

http://www.wavelengthaudio.com/usbdac.html

Specifically, the preamp (or audio interface) needs to connect components TO the computer via a USB, or better yet, a FireWire port. Hagerman make something similar to this but the USB port is an input. I believe the same goes for the Wavelength.

The Wavelength Cosecant DAC is a digital to analog converter with USB input. It can be made with a volume control which would turn it into a DAC/PRE.

Bel Canto Dac3

I don't own one and I am not a dealer but I have an acquaintance at the local audio shop and he has one. Seems to like it.

I think Wavelength offer something like this.