USB DAC's -- the best

Cosecant USB DAC v3 (now with ASYNC mode)

How does it sound compared to a Nugent modified DAC 1 USB

Some clarification: It is not necessary to send in your laptop for the Spoiler DAC. The Pace-Car does reduce jitter a bit over the standard USB interface, but it is optional. If you want the lowest jitter, then go with the Pace-Car. If you use a WiFi server such as the Squeezebox, then you can use the Pace-Car with Spoiler option without sending in your laptop.

Steve N.
Empirical Audio

Look into the new async version of USB DACs as this seems to be the most noteworthy improvement made in the past couple years. Wavelength offers the async version in the Brick & Cosecant. Not sure if there's an async option for the Crimson.

I used to own a non-async Cosecant and changed to a non-async Ultra Fi Times iRoc in 2007 which I'm very happy with. In my system it was a noticeable improvment as the dynamics and bass were improved over the Cosecant. The dynamics and bass improvement could be the result of the current-to-voltage output design as the iRoc has very low impedance compared to the Cosecant. I'm not an EE, all I can tell you is what I hear.

Ultra Fi Times is about to release the async USB DAC and the preliminary reports from people that have heard it are quite impressive. You may want to wait until the dust settles a bit, but it sounds like you want to go the async USB route.

I have really enjoyed Headroom Desktop Amp. I know that there are other 'pure' DACs out there that get better press and reviews, but I have been utterly impressed.

I think a worthy rival could be found in Steve Nugent's "Spoiler", especially if internally clocked with his "Pace-Car". You'll have to send in your laptop, so that Steve can synchronize the computer's clock with the clock in the unit. The result on a high resolution system rivals, if not even surpasses the sound of a Zanden combo or a dcs chain easily.