I would recommend the Yulong Canary. You can get it now for only 258 US. It comes in black, silver, or red. It has the awesome ESS9018K2M Sabre DAC in it, and the amp has plenty of power output to drive the K712 Pro. This will be a very nice upgrade from your Z card. You will be happy.
http://www.ebay.com/itm/Original-Yu...hash=item1a217eb9e6:m:m3Iv0O6i42CK4rumZYGPCuA
I was actually looking at that, and almost was going to jump on it until I found out it apparently has a Output impedance of 10 Ohms according to thread below? Which if you apply the 1/8 rule means it is suitable for headphones 80 Ohms or higher i.e. not good for K712 with 62 Ohms. Is that right?
https://www.head-fi.org/threads/daart-yulong-canary-dac-amp-based-on-es9018-k2m.832671/page-2