NAD M51 upgrade

Been running a NAD M51 for a few years now and have been happy with the performance. I tried out a RME ADI-2 FS and found it too be too bright, and that it did not match the musicality of the M51. Any suggestions on a DAC upgrade? I'd like to spend <$2k. I'm using the NAD M51 as a preamp as well so I'd prefer a DAC that has remote volume control. Mytek, Chord and Metrum all seem to be highly regarded.
mytek and chord will still be sharp sounding in the same school as the rme you have tried

for a smoother fuller, richer, more analog tonality i suggest

mhdt canary, orchid or stockholm
ayre codex
metrum (any model)
van alstine tube dac (there are various generations)

nad gear will tend accentuate any sharpness in the signal

good luck

The RME is not bright but tells it like it is. You’re going to chase your system forever as it’s probably just bright and that could be your room is the problem, the music you enjoy is just poorly recorded or your speakers are bright or like the RME just tattling on your room and gear. Curious what the source is you’re feeding he RME, cables and what you’re using for streaming such as basic Spotify and etc.?

As mentioned above the other DACs you are questioning are transparent as well and will not sugar coat things. Want the best performance out of the RME then use quality cables and if you can upgrade the power supply. Even with the stock power supply, with a really good power cable the RME will perform. But no the RME will not roll things off to fix problems.
Recent MHDT Orchid purchase is filling my bill for smooth and rich. Sharpness is minimal so far.  So dependent on other system ancillaries. Smoother than Wyred or PS audio for me, but its a bit tubey and has one tube. Transparency can be a fine line for me.   
I have not compared to the NAD m51. Bit running silver DH labs interconnects out of the Orchid. Wish Orchid had XLR. 
Amp = Job 225
Speakers = Ascend Sierra Tower w/ RAAL tweeters
Sub = Rythmik F12
Source = Bluesound Node 2 (Tidal)
Digital Coax cable = Cerious Graphene
Speaker cables = Tara Labs RSC Prime
Interconnects = VH Audio Pulsar Copper
Power Cords = custom made

If the coronavirus were not in full effect I’d hit up some audio meetups in the NYC area and demo some gear. Note that there is nothing bright or harsh about my current setup. The M51 isn’t as resolving as the RME and I found it to be more musical than the RME. The RME was definitely a step down from the M51 from a pure enjoyment perspective. It just sounds, mechanical, like a piece of dry studio gear. I’m not trying to mic an instrument to get the absolute greatest detail so I can carve up a mix. I just want to listen to music.

Has anyone that has heard the M51 found it to be rolled off on the high end?

I had an MHDT Paradisea+ many years ago but haven’t heard their newer models, plus they don’t have volume control and I have no desire to purchase an analog preamp as I’m all digital.
metrum jade has remote volume, if you can find one

sonnet morpheus too, but it is over 2 grand

ayre codex has the sound you seek, it has volume control, but not remote
Thank you for the tip on the Sonnet Morpheus. I think that and the Lumin U1 Mini will be my next upgrades.
One other comment. People seem to think that DAC's are the end all be all of sound. Let me remind you that Analog is still king in the studio world. Analog synths and effects gear using tubes (gates, compressors, summing amps) operate in the analog domain. The BEST studio gear is analog. Tape compression still cannot be duplicated in the digital domain. 
I recently replaced an M51 with the RME and I think it is a much better DAC. The M51 is no slouch but it simply is not as clear and intelligible as the RME. CLARITY, would be the word to sum up this DAC. Much better delineation of the lower registers too. It is not bright at all in my system , in fact it is perfectly balanced from top to bottom. The feature set is off the chart, its the Swiss army knife of DACs