EverSolo DMP-A8 with Denafrips Pontus 15th, both have i2S

As has been pointed out; if the clock in the DMP-A8 isn't as good as that in the Pontus, or indeed a Denafrips DDC, by using I2S out of the streamer in to the Pontus could mean you maybe actually losing performance not gaining any. 

Whether the clock in the DMP-A8 is better that those in the Denafrips DDCs, or the Pontus 15th I suppose is the question 

Well, I guess for the short term I should use USB from DMP-A8 to Pontus 15? That is if I can't hear a difference? I appreciate all information, whether it's good news or bad.

What should I be listening for, to compare two different DDC’s. I know this is probably a loaded question, and not sure if there is a good answer. Just reaching out and trying to learn.

Why would you want a DDC?  Use I2S from the A8 straight to Pontus.   Less is more!   In my case, I'm very happy with direct I2S from my A8 to a Harmony DAC.

If the clock in the A8 is a femtoclock then it’s not as good as the ddc.  Likely a toss up to the clock in the Pontus.  The ddc clock, at least in the Hermes and above is an OCXO (oven controlled crystal oscillator, which) clock which is much better.  The Iris uses a TCXO (temperature controlled).  The control of the temp of the clock greatly increases performance in terms of reducing jitter.

Comparing ddcs I would listen to the aspects that were mentioned previously, soundstage, imaging, transients, bass response.  Even treble and midrange response can improve with a ddc.  Noise floor is also lower with the ddc.  You really just need to listen critically and decide what you like but certainly pay attention to the things listed above.