I've heard the D'Agostino momentums in the same room as Luxmans. I have to tell you that you should listen to both. :)
The new Krell Solo XD amps are a completely different sound than your Evo 600 generation. I think the Evo generation was the best Krell amp in their history. The new Solo and XD type amps have more of a very smooth Class A sound. Really smooths out the sound and gentles the highs. They are definitely not as crisp and neutral as the Evo 600.
I think the new D’Agostino amps are voiced warm, not like Evo 600 at all.
I think the MBL and Gryphon are going to be the closest in sonic signature to your Evo 600. Another option would be Soulution, which I thought sounded really high resolution and transparent like the Evo 600 were (but they are seriously expensive).
I don’t necessarily agree with those dealers that say there are better options available "nowadays".
In the end, it’s up to you if you want to find something with the same sonic signature or something different.
I have heard Boulder amps. While they are not as smoothed over as the new Krell XD stuff, they do still have just a tiny bit of the lush coloration that is typical in some of these bipolar Class A amps. The EVO 600 is still more neutral and uncolored (some people don't like Krell). No wrong answer here because it's all about personal taste.