The better amp?

joser - TPA6120A2 is a monolothic "chip" type op amp headphone driver.  Like mijostyn said, discrete analog stages will almost always sound better than monotholic op amps.  Also, Roland is driven by a wal-wart switching power supply, which is never as good as linear power supply (like in Emotiva). I have said this in your other thread.

@auxinput, thanks, but I just want to know between those 2 amps and the Roland uses USB power, is a audio interface via USB.

@auxinput: Which of the specifications of both amps do you see best for audio quality? I wrote the link for each one of the Texas Instruments page.thanks.

@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.