Could you recommend a nice used USB DAC?


I am looking for a nice used USB DAC.  I currently have an Arcam CD23 which I really like to but I am looking to go with the USB DAC route for high resolution streaming. 
Could you recommend one and give a short description on the sound?
It seems like there are a lot of recommendation for the Chord Mojo DAC but it seems to be a bit more on the "detail" so I am not sure it would be for me.  The reason I like the Arcam because although it has a lot of details, the musicality always comes first.
andy2
if you looking just for a USB input DAC, go for BorderPatrol DAC, its flying under radar, and very rarely you will find it for sale used anywhere online. Ask any senior / seasoned audiophiles, it is as good as $5-7k Dac for just $995 msrp. 

I used to have that Cd player years ago. The FMJ, full metal jacket. My sister bought it from me, I now use the Aloia for cd's.
Scott Nixon usb tube dac new is what I'd recommend.  I’ve been using Nixon tube dacs for years and they’re very affordable. I've never heard anything better. Legends...

http://www.scott-nixon.com/dac.htm
I just put in an order for the Ayre BQ-9 DSD.  Hopefully will get it soon. 
Finally I received the Ayre BQ-9 DSD. Here is my short impression and also compare bit with the Arcam CD23.First the Ayre is supposedly has its own USB power supply.  And also as important, the USB data line is optically isolated so it seems like a well careful design to shield itself from the noise of the PC side. Therefore the quality of the USB cable is probably not as important.  My impression is limited to red book cd only at 16bit/44khz since I do not have the software to play hi res audio.

First I really like the Arcam. It has both the details and at the same time sounding very musical - not an easy combination. Even though it came out around 2003, I still think it may be the best one box cdp at its price even now.

Most people have said cdp for the most part sounds slightly more musical vs. PC played through the USB DAC which is a bit more cleaner, analytical. I don't know whether it's a general truth, but I do find the Ayre follows the same line vs the Arcam. The Ayre bass is slight tauter, cleaner, may be slightly faster. The midrange is also slightly leaner or clearer (however you will). The very top end is a touch slight more open.
But on the other hand, I still prefer the Arcam. The Arcam is just slight more musical, more delicate and a touch sweeter. Through the Arcam you're just more there with the musicians. There are slightly more air and the instrument sounds a bit more natural. The Ayre on the other hand sounds cleaner and may be a touch more dynamic at the bass.
Other than that, both actually sounds somewhat similar. There is nother forward or harshness.

Hopefully I get to play some hi res on the Ayre soon.