DAC Break In Period

In February, just before this Covid storm settled in, I purchased a Hegel H390. I couldn't wait to get it home and get it hooked up. Right out of the box it sounded much better than the class D amp I replaced. After several hours of listening it even sounded better.
Now, back to right out of the box. I first hooked up my Cambridge Audio Azur 651C to the analog connections on the back of the Hegel. The sound was incredible. Sound stage, imaging, dynamic sound and not harsh at all. I then hooked up the 651C to the Hegel using the Coax and used the Hegel's AKM 4493 DAC. What a difference! It was horrible! I tried different CDs, different genres and different volumes. It didn't matter. The DAC in the Hegel was awful. It was harsh with a very limited soundstage. I decided that I would only use the DAC in the Hegel for when I'm playing background music using Airplay. I'll do my critical listening with the sweet sounding 651C.  I've done that for a few hundred hours since February.

Earlier this week I decided to try the Hegel hooked up via the coax again to the 651C. I have been completely blown away with how good the DAC sounds now. The first thing I noticed was the width of the sound stage and how the images in that extended sound stage were very palpaple. I was listening to Amber Rubarth's Sessions from the 17th Ward, there was a guitar being strummed 5 feet to the right of my right speaker and it sounded as clear and defined as an instrument located in the center of the sound stage. The sound is rich and it's not harsh at all. It's very dynamic with detail that I never got from my 651C.  I've heard of people speaking of multiple harmonics coming from a bass guitar strum and how hard it is for a system to do that justice. With the Hegel H390 I can now finally hear what they were speaking of. I am powering Focal Aria 936 speakers and I suspect better cables might even improve this more. (I'm using all Blue Jean cables now.)

I've been listening for three days now and I've tried multiple CDs and genres and the results are always the same...phenomenal!

I guess my point in all of this is, don't give up on a piece of equipment because it doesn't sound good right away. Sometimes it really does help to burn it in! I'm so happy I went back and tried the DAC in the H390 again!
Of course you know this is all expectation bias. Or some kind of bias. You listened to a lot of different recordings?? Not the same one over and over again? Meaningless! Auditory memory only lasts a millisecond. No one can hear these differences. What measurements do you have to prove this? Where's your double blind study? Were the connections cleaned the same each time? 

Sorry. Thought I'd just get all the usual BS out of the way. Yes some components go through major changes as they burn in and warm up. Seems to be even more so for solid state and chip based components like a DAC. As you so clearly have noticed. 
The Amber recording ( and music ) is fantastic:-)
of course the science denial bunch won’t understand why.... head transfer function 

what optical cable are you using ?

enjoy the music and the journey
Djones51, I’ve only used the 651c as a transport and apple airplay through an Ethernet connection.
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Tomic601, I’m using Blue Jeans Cable with RCA connections. Coax out and coax in.
Roger that. I have some Blue Jeans cables, competent and very well constructed at super reasonable prices... but.... you are I believe leaving a lot on table sonically- for example try a NOS Nordost Silver Shadow SPDIF cable....

@buff here on Audiogon might be able to help you - I have bought more than a few from him, excellent cable and sonics :-)))

have fun, enjoy
Djones51, Apple Airplay through the Ethernet  connection does utilize the internal DAC of the Hegel and I played that for many hours
A different S/PDIF coax can be a major upgrade. I auditioned cables for about a year.

it not only sounds great, now you don't have to read the endless debate over breaking in a DAC...I agree it's time to try some different coax cables...
I’m hoping Santa will set me up with a nice coax cable. I’ve been good this year!
@baclagg-     KUDOS (for finding and acknowledging that reality).      Out of curiousity: what would it take to convince you, "It’s all in your head!", now?                                        Happy listening!

I’m so convinced now I believe I’m moving on to cables and see what that does for me!
Check out either the Kimber Kable or Synergistic Research offerings (even older/used), if you want to preserve your music's nuance, sound stage width and depth (etc).       It's surprising (in a dismaying way) how many interconnects will act like a blanket, thrown over your speakers.     Happy listening!