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

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.

@mbolek makes a fair point, a ddc is often used to clean up USB signals.  That is why I have one as I stream using a MacBook Pro.  Given that your streamer has I2s output you may not gain much with the ddc.  You would be getting s much better clock with the ddc but then using a different output from the streamer which would involve conversions to I2s format when outputting from the ddc to the dac.  The clock might be worth the effort though, timing is significant in the digital world.