Gustard R26 - different sound signatures in different units

The Gustard R26 was recommended to me this year by reputation through a guy who hadn't heard it but knew people he trusted who recommended it. 

Now I've listened to a few R26's now with mixed results. It's been a journey and I made some dumb mistakes by not paying close enough attention. 

The first R26, new from Amazon, sounded dull and uninteresting, not extended on top. I was driving it through USB. I exchanged that R26 for another one, which sounded similar, but then I discovered driving it from the U18 DDC through coax (I have a very good coax digital cable) and the sound signature was totally different.. lively and extended on top. 

Then I found a used R26 on EBay for much less than I had paid for the new one from Amazon, so I returned the one to Amazon and purchased the one from EBay. It sounded brighter than the one I had returned, but at first I didn't mind. Maybe was more detailed. Strangely, the display had a micro font compared to the first two I had.

For a time I didn't do any serious listening. I was more involved in my piano playing and composing hobby. I eventually went back to the R26 I got from EBay and realized it was much too bright. I wondered if it had old firmware, given the micro font on the display. I tried upgrading the firmware, but there was no change in the display or sound.

I ordered another R26 from Amazon. This one sounds completely different than the one I got from EBay. It has about 120 hours on it so far. The problem is that it has shrill/unpleasant sibilance. I'm not saying it's particularly bright. The R26 from Ebay is brighter yet doesn't sound shrill. So the this new R26 probably still has some burn-in time needed, but the shrillness hasn't diminished at all over the first 100 hours. Maybe it's even worse.

Anyone have a similar experience with any of these?