Best Dac under $600... New or used

Gustard x12 dac might be worth looking into. It can be purchased for under $600 new with shipping included and a 14 day money back guarantee from Vintage Audio Lab on Ebay.
I bought the Gustard u12 usb/spdif converter and it has make the biggest difference in my SQ with Jadis and Art Audio amps and Audio Note dac.
Gustard is coming out with their "flagship dac", the X20 pretty soon, so I am very curious to hear the reviews on it.
Here is some copy from an Ebay ad for the X12

Gustard X12 ES9018 Sabre Asynchronous 32bit 192K USB DAC with XLR Balanced Output

2015 new version with DSDx64(2.8224MHz) DSDx128(5.6448MHz) support --- through USB port and DOP64 through Coaxial RCA/ AES and Toslink Posts

ESS Technology Sabre Reference 32bit ES9018 is best performance dac chip on market. This chip is not a audio friendly chip which will product very aggressive analytic sound on optimised circuit design. So it is quite a challenge chip for dac designer. This is most musical es9018 based dac we ever heard. You will like its ultra transparent, high resolution and vivid analog sound which is quite close to high quality vinyl record.

Features:
1. Update version ESS Technology Sabre Reference 32bit ES9018S (new version) dac chip
2. Two sealed high grade transformer deployed, one for digital, another for analog circuit.
3. XMOS X-CORE 32bit USB receiver chip which is best performance asynchronous USB audio solution.
4. Discrete transistor based balanced analog output stage for extended dynamic and analog sound

5. Precision 100 steps digital volume control; from 0db to 99 db attenuation, 1 db per step

6. Vishay round SMD precision resistors and carefully chosen parts for best sound performance.

7. Professional circuit layout to get best circuit stability and great performance.

8. Aluminium housing, hair skin surface procession.
9. Digital Input: balanced Spdif AES/EBU port, RCA, Toslink Optical input and USB
10. Analog Output: balanced analog output via XLR port, single ended output via RCA port.
11. Dimension: 190 x 70 x 313 mm (W x H x D)
12. Both 110/120v and 220/240v version supported.

Digital Input Format Support：

Coaxial：

PCM：16-24bit /44.1、48、88.2、96、176.4、192 kHz

DOP：DOP64

optical:

PCM:16-24bit /44.1、48、88.2、96、176.4、192 kHz

DOP:DOP64

AES/EBU：

PCM:16-24bit /44.1、48、88.2、96、176.4、192 kHz

DOP:DOP64



IIS:

PCM:16-32bit /44.1、48、88.2、96、176.4、192 、352.8、384kHz

DSD:DSD64、DSD128

DOP:DOP64、DOP128


USB： 16-32bit / 44.1、48、88.2、96、176.4、192、352.8、384 kHz
DSDx64(2.8224MHz)、DSDx128(5.6448MHz)

Analog Output：

RCA Output ：2.5 Vrms @0dBFS

XLR Output ：5 Vrms @0dBFS (XLR format：1: Ground 、2: Hot、3: Cool)

Frequency ：20-20K Hz +/-0.15dB

Update driver for Windows XP/Vista/7/8 can be download here:

http://pan.baidu.com/s/1ntNXwN3

Mac OS X 10.6.4 or newer version do not need external driver, just plug and play.

DSD Plug-in downlod here:

http://sourceforge.net/projects/sacddecoder/files/?source=navbar

How it sound?
Keep this section empty for critical audiophiles.

Shipping worldwide: This dac will be shipped by EMS air express with insurance and on-line tracking.

Combined shipping fee for multiple items of all my products is welcome and will save you lots of shipping fee. Please query me before your bidding.

We have plenty of stocks of this product. If you need to order more than one set of this product. Please query us, we will create a new list for you.



Wholesale price offered for bulk order.

I'd also look at the Parasound Zdac V.2. It has the volume control you want, tolink, co-ax and USB (all asyncronus) as well as dual headphone jack.

X12 channel swap (DSD playback from USB input)
I got information that the manufacturer was aware of this issue but had no plan to fix it (as of today).

I know it's not below $600 - but MusicDirect currently has the Musical Fidelity M1SDAC on sale for $699 with free shipping

Regards...