I've used external DAC's since early 90's for all kinds of music as hifi and to mix. And current one also doing 24-bit and 96k I had since 2010.
What matters a lot are capacitors and quality of those - very expensive if to use the best kind.
I'm overly surpriced how much of a difference it makes.
One of the few A/D converter shootouts I found:
http://www.kvraudio.com/f...ghlight=test+converter Some chips on various equipment:
https://www.gearslutz.com...d-da-chips-listed.html One way to test is to make tests with RMAA Rightmark Audio Analyzer, might give a hint.
http://audio.rightmark.org/index_new.shtmlIt also list some interfaces and their test data.
Run test patterns from disk or a CD and see what results are.
You can do loopback test first with your interface in and out - and do same test pattern on your CD/DVD player etc.
But it's all about ears, really, numbers only give a hint.
More about DAC's then.
I bought a $100 DAC based on CS4397 from HongKong - replaced onboard capacitors according to this guys tests:
http://www.lampizator.eu/LAMPIZATOR/LAMPUCERA/CD%20DAC%20Lampucera%20lampizator.html A DIY with picture but text in swedish I did long ago for my current DAC cost total about $200 to make.
http://www.faktiskt.se/phpBB3/viewtopic.php?f=3&t=35665&p=1760579#p1760579 Lampizator bought 5 of those boards, and experimented with different caps and presented his own grading of sound. This guy is building DAC's that starts at about $1500 or so, but tube stuff since for music listening and not mixing.
Cyrstal Semiconductors has been among the best since long, but it is all about total build and chosen components.
Overall, in my previous DAC and current - the double ended outs to a differential op-amp are really difficult to get good sound if. So simplest solution is to use single ended outs, and skip op-amp. I'm sure there are good circuits but didn't find one myself. It depends on DAC circuits you use, if there is built in filter with external components, you can do single ended. Others have the full filter externally and you cannot do it that simple, the sample rate filter needs to be there.
I just put Sanyo OSCONS that cost about $5 each, and then Tantal 10 uF on top of those.
Tremendous lift in audio quality. I run spdif out from RME cards to this DAC for mixing.
Have fun....