[AbedFa]

So the title says it all. I have the following setup:
-Cayin C5
-Fiio X3ii
-JVC HA-SZ1000
 
I'm loving it, the Cayin C5 is simply amazing, but the short battery life is a big issue to myself since I use it a lot, I often charge it 2-3 times a day, and there was a few times when I used it while charging (I know it's not good for the device)
 
I looked around for a USB DAC for the same budget range as the Cayin C5 but the DACs I found are quite more expensive, and I wasn't sure how do they compete in comparison.
So here I am gentlemen, humbly asking for a little assistance. I don't wanna kill the Cayin C5 by using it while charging.

 

[fishyee]

Check out the CEntrance Dacport HD or Slim (if you can find the Slim on Massdrop).  Your headphones are not hard to drive so any decent USB dac/amp should do.  Perhaps the Apogee Groove too.

 

[willsw]

Just to clarify the language, the amp is what is driving the headphones. The Cayin C5 and Fiio E12 are amps, the DAC you're using is in your X3ii. 
 
Assuming your price range is $100-150, you can find plenty of USB DAC+amps used, here, or on Audiogon, or usaudiomart.com. You'll likely not find a USB DAC+amp with bass boost, as if your source is a computer you can just EQ your bass boost. 
 
Or just buy another Cayin C5.