Which DACs are known to be sweet/rich/relaxed?

Hi Robert, how did you conclude that it is the DAC problem?  Might be the amp and speakers being too revealing like what chayro said.

Perhaps a Conrad Johnson or Acoustic Research tube preamp would be enough to "flatten the curve", so to speak.

:-)

If rich and relaxed is your preference, it is hard to beat Audio Note.  They have terrific sounding DACs, although the prices on top models are quite scary.  The less expensive models are more of a bargain--they give you much of the Audio Note sound, with only a little sacrificed compared to the very top.  I heard a three way shootout between a 3,4, and 5 (sorry, I don't know the particular models within these three basic designations), and I liked the 4 because it gave one most of what the 5 offered but was not as crazy high in pricing.  I did get to hear the 6 and its companion transport, but, I only heard it in a setup used to test and burn in those items before they went into a super system.

I would also endorse the recommendation to at least look at different servers.  It seems a bit unbalanced to have your kind of components and not have a high end server.  It is a bit of a tricky matter of system matching--I know someone who hates the Aurender in his systems--so it might make sense to work with a dealer with options and the technical skills to optimize the system.  A local dealer is really big on specialized switches for his network.  Some day I will borrow the switch to see if it makes a difference (he uses Ansuz).  I don't have a personal recommendation because my server, DAC, CD ripper and storage is provided by a single brand that sort of combines these things (NAIM 555 server/Uniti Core ripper and NAS).  

All in all, there's nothing wrong with your total system. Rock recordings played at HIGH volume is your REAL problem, most of this genre pretty poor recording quality, compressed, no sense of natural timbre. I'd say your system was reproducing these recordings exactly as it should. I don't know that any of my or previous suggestions will help, except those for less system resolution.

Loud rock music played at loud volume is best left for live concerts at large venues.

So I've now listen to a Nucleus vs Innuos and found VERY little difference YMMV. I kept the Nucleus and am very happy.

Uhm what? 25 to 40k budget for a dac?

All you need is a 1200-2000 dollar border patrol dac.

The OP has a top class system, I doubt he wants to go from a DAC with ~22 bit resolution to one with ~10 bit resolution.