The better amp?

@joser9616 
Problem is,  there is not straight answer. I have several different op amps at home now, including the 21#4.  I've listened to the 6120A2 driving headphones with good results,  but I've never used it.  The 2134 is a versatile op amp,  it works and sounds good in many circuits.  
Depending you what you are using it for either one might be better than the other.  You need a host of spec's of what it is being used with to get a decent handle on paper to give you an answer and in some cases,  it could end up being only your taste that tells which is better. In most cases the manufacturers do a good job of selecting op amps.  If you want to roll op amps,  I would look at which op amp is in the unit and look for op amps with similar input impedences making sure minimum and maximum voltage capabilities are within the operating limits needed. There are many other things to consider,  signal to noise, gain, fet/ bipolar etc. 
There isn't a straight forward answer,  You are going to have a hard time to get an answer without posting a lot more info and even then.  
If you really have no idea of what or why,  I suggest to leave it alone or be prepared to purchase several to try.  Good luck,  Tim 

USB contains 2 wires for digital signal and 2 wires for "USB Power".  This is just 5V DC and it's driven from the computer/laptop (which is basically the same or worse than a wal-wart switching power supply). 

Also agree completley with timlub.  You can just compare op amp specs and expect it to tell you exactly how good a DAC/preamp is.

In comparison, the Emotiva DAC has extremely high current power supply that has a +12V and -12V lines to the op amp / audio circuits.  This is much better than another DAC that uses switching wal-wart or only a 5V USB power supply.

@auxin @timlub, thank you guys for the info. I'm just learning here... 

I made a comparison between the Emotiva XDA-1 and the Roland Mobile UA and I am very serious about it, the Mobile UA is much better than the Emotiva, a lot more detail, more clear ... that interface can be connected to studio monitors In a balanced way, it is exactly the same as connecting it to my two Crown D-75 amps, the active studio monitors(active), as you should know, have their own amp, I have the amps alone with speakers connected.