DAC upgrade suggestion

I am currently using a Audio-gd nfb 3.1.  The information below.  I bought a Topping DX7 pro based on the good data in audio science review.com.  When I first use it, I can hear the difference immediately.  It is more dynamic, bass is more punchy, sound clearer.  It sounds great.  But after listening for a longer time, I feel a bit fatigued.  I can compensate it a bit by switching the tubes in my amp.  Then, I need to return the DX7 because the Bluetooth doesn’t work.  I plug back in the Audio-gd, and i can confirm it is not fatiguing.  It is less dynamic, but also smooth.  Now, I am confused on what I should upgrade.  Several Audio-gd DACs are reviewed by AudioScienceReview.com and the data is very bad, LoL.  I know the bad data doesn’t mean it sound bad, but should I avoid DACs with good data?  How should I pick my next DAC?  It seems R2R DACs maybe a good choice for me since many people says the sound is more analog-like.

Out of curiosity, I search for review of the NFB-2 and found the one below.  I am surprised, it is exactly how I feel about it.  It also mentioned that the designer is aware that it won’t have good test result due to non-feedback, but it will sound smooth and good to human ear, which also seems spot on.  Hmmm after all, it seems the designer know something, LoL.  
If I like this sound, maybe I should stay with this brand, talk to the designer and see what is the best upgrade option that improves the dynamics and clear out, at the same time keeping the same smoothness.

R2R dacs don’t measure as well but DO sound better !!  At least the ones I heard..  Try the Schiit Modi Multibit or the bifrost..  I have the Yggdrasil and the modi multibit dac.. both are excellent sounding and make digital sound like analog music !! 
It is more dynamic, bass is more punchy, sound clearer.  
It seems this isn't what you're looking for. R2R DACs or maybe the RME ADI-2 which has some EQ capabilities would be more to your liking. 
I bought a Topping DX7 pro based on the good data in audio science review.com. 

Those guys base every recommendation on measurements and not listening.
Actually, I like more dynamics, punchy bass and clear, transparent sound.  But the tricky part is, to a point that not to sound fatiguing.  It is more a feeling after a longer time of listening.  When I first hear the Topping dx7 pro, it sounds great.  But after maybe 30 mins, I feel it is too “noisy”, and I need to turn down the volume from my normal listening volume.  I guess this is called the synergy between the components.  I am using a Triode Lab 2A3 and Spatial Lab X5.  Both of them are very revealing.  It seems they match better with the Audio-gd.  I also try the Almarro 318 with the Topping, I don’t feel fatigue.  But I like the Audio-gd + Triode Lab More than Topping + Almarro.  
I think I have two top candidates:

1) MHDT Orchid
this has very good review by reviewer and the community.  People often comment it has a smooth, liquid sound.  
2) Audio-gd R7 or R8
this has rave review in 6moons with a blue moon award.  It is the same brand that I like, and hopefully with similar but improved sound signature.  Also, it is very flexible that I can choose a TDA1541 smoother sound, and NOS vs with over sampling.  
Forgot to mention, the Audio-gd has a in-house developed R2R chip, while the MHDT use an old TDS1541a.  Although the TDA 1541a has good reviews, but if Audio-GD do it right, a new technology should theoretically sound better.  And with the positive review in 6moons, I hope it means it is done right.  This is another pro to go with Audio-gd.
I will give my vote to MHDT Labs Orchid.

I have three DACs, the one I already mentioned and a Jolida glass FX tube DAC III and an RME ADI-2 DAC FS. I personally prefer the Orchid out of these, it's very smooth non-fatigue sounding in my system.

Just my 2 cents :-)
The Yggy is amazing - and it gets superb reviews, which rank it with the best available. 

But, the Dennafrips Terminator and Audio Mirror Trubadour also get superb reviews.  

All these are R2R Dacs, which sound much, much better than other types of Dacs.  

If you want sound, which competes with the best, these are the Dacs to consider.
I'm in the same boat as the OP. Just got a Topping D70.  It's definitely more detailed than my old MyDac.  However, I too, find it a bit harsh after a while. Maybe I just like bad DACs......
I have the Audio-gd R8 DAC and its a superb DAC for the money. You also have many setting options on the front panel and its FPGA programable wich can upgrade its sound with new firmwares that they launch about twice a year.

Ive heard three systems much more expensives than mine (just the DACs costed about 15/20k) and I prefered the sound of mine by far. Of course the whole system is important, but the R8 played its role with authority.

A couple of advices:

1-If you use USB from computer, upgrading your digital rig with an audiophile switch plus (like the Uptone Etherregen) a dedicated streamer (like the SOtM SMS-200 Ultra Neo) plus a good DDC (I have the Audio-gd DI-20) will upgrade the sound quality of your DAC to a compete with ones costing many more times. 

2-Dont pay attention to measurements, just your ears. My Line Magnetic LM-508IA 300B 805 valve amp has about 1% of total THD and my NC500 based class D amp (well regarded among audiophiles) has about 0.001% of THD. I prefer by a WIDE margin the LM-508IA, its a music monster. Much more realism, body, 3D sound and tone. Numbers are just that. You cannot guess the taste of a food for its calories. 

Finally, you can read a good review of the R8 here: https://www.hifi-advice.com/blog/review/digital-reviews/spdif-dac-reviews/audio-gd-part3/
In this review the used the older version of the R8. Now it has new DA boards and new firmware that rise the sound quality considerably. 
I have an FPGA dac from PS Audio and it sounds analog to me, and measures so poorly that the Audio Science review guy gave it an "award". 
Right on kairosman! I have a chord qutest and it sounds better than any dac I’ve had, including r2r. Very analog and future proof. Best deal in dacs today imo. 
If you want one Dac to enjoy music for a while, rather than churning through experiments, get an Ayre Codex.
R2R DACs are ridiculous - no two resisters are the same in the real world.
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@gte357s  - sorry if another asked and i missed the response, but what is your price range? I just went thru a full DAC buying cycle. 
+1 @ erik_squires. I just upgraded from a Node 2 to a Mytek Manhattan w/ network card and could not be happier. Have listened for hours on end without any fatigue. Didn't realize digital could sound this good.  YMMV. 

Yeah, I was going to take advantage of the Mytek trade-in offer when Covid hit. :(
If you want a “great” dac , who’s so natural, holographic, pure class: the Metronome Technologie C5: transistor; C6 : tubes or the C8: both. You can switch between tubes and transistor. Never heard such a dac before!
Thanks for all the suggestions.  Some of them haven’t heard before, Will am have a look.

My budget is < $1500.  I am ok to buy used, but those popular ones seems hard to come by.  People are hanging on to them.  =)
I love my Border Patrol SEi DAC.  I don’t care how it measures.  It makes my ears happy.
For the price around 1500$ : you can listen to the PS Nuwave, or (he is very good): Teac 501 or 505. Enjoy your music! Lukaske.
I also seriously look into the Audio Mirror Tubadour III.  It has good reviews, and it is from US.  I have bad experience in the Topping DX7 purchase.  So, I am a bit reluctant to buy from China.  I don’t know if the courier services like DHL resumes or not.  Shipping from US is a big plus.  
I’m pretty happy with Mytek Brooklyn DAC+ and Bluesound Node 2 (just for the streamer). I don’t know how the others compare and would love to do a bunch of a/b, but at least no fatigue here.
Line Magnetic LM805ia amp. 
Another + for MHDT Lab DACs, but its seems to me you should be looking at their Pagoda instead of the Orchid. The Pagoda uses dual PCM1704 chips which are capable of processing 24/192 without down-sampling, unlike the Orchid. The Pagoda presents a natural, detailed, yet fatigue-free sound (not "romantic" or "tubey" at all). Replacing the stock tube with a WE396A (D getter, JW military version) elevates the sound quality quite substantially IMO. Best of luck on your search. 
If your budget is ~$1500, consider the Schiit Gungnir MB. I've been quite happy with mine. Very natural, analog sound. Still lot's of detail.(if that's even possible) :-)
If you want one Dac to enjoy music for a while, rather than churning through experiments, get an Ayre Codex.
R2R DACs are ridiculous - no two resisters are the same in the real world
Ironic given that the resistors on-chip in the ESS DAC in the Codex are far more dissimilar than in any R2R DAC I've seen.
+1 Ayre Codex

Due to system synergy (mine is conrad johnson mf2500 feeding thiel 2.7) YMMV -- after trying PS Audio Directstream Jr, Nad M51, Chord Qutest, etc........ then an Ayre Codex I am done!   I have not tried the R2Rs -- so maybe I am missing something -- but will likely never know.
I to went thru a Audio Science Review phase and started questioning the sound and build quality of my Audio-gd pcm 1704 DAC. I thought there must be something wrong with my ears how could I prefer the sound of this DAC when ASR clearly shows that it has engineering flaws?

First, ASR has never posted a test on a Audio-gd DAC with DSP-1 or FPGA processor(augments the difference in resistor tolerances) as far as I know.

Second, the test they did on Dac 19 DF amirm states he does not know if jumpers are even remotely set right on DF digital filter but proceeds with test anyway.

Third, ASR/amirm list specs for DSP-1 version specs claiming their test on DF clearly shows that Audio-gd does not achieve test specs listed on their site.

ASR/amirm and posters do make some valid points, mains wires and RCA socket wires should be twisted but for the most part IMO they are justifying their purchase on specs alone not sound quality.

All of these posters rave about how superior the R2R DAC's are....yet I suspect none of them have heard Lampizator's  DS53 digital engine, a derivative of the digital engine used in their Pacific DAC. They’ve standardized this Delta-Sigma design throughout their line, and to my ears it's sonically superior to R2R designs.
I think your system determines what DAC sounds good with it.

My Thiel CS5's, yeah they are old speakers, can sound a bit piercing in the treble if one is not careful with equipment matching. 

I tried several DACs in the $1K to $2K range and they seemed a bit "tizzy" to me in my system. I settled for a Hegel HD25, which came close, but it still was not quite right.

Took a chance on a old Theta ProBasic III R2R DAC for like $500 about 18 months ago.Sounds great to me with a PS Audio PWT. Not sure if it sounds more analog like, but to me in my system, it is a bit smoother with no loss of detail. 

Thanks for listening.
Audio gd just look at the forums digital is supposed to run cool not gd ,they run hot and horrible distortion specs look at audio science forums and plenty other board failures thats why I bought a Denafrips runs cool and exceptional low distortion .
@three_easy_paymentsJust listen top what 

"All of these posters rave about how superior the R2R DAC's are....yet I suspect none of them have heard Lampizator's  DS53 digital engine, a derivative of the digital engine used in their Pacific DAC. They’ve standardized this Delta-Sigma design throughout their line, and to my ears it's sonically superior to R2R designs.

Yes-- to their ears, R2R DACs are "superior"-- to your ears, the Lampizator Delta-Sigma is "superior". This "proves" nothing, except that each person hears differently, has different concepts of what sounds "best" to them, and is most likely listening on different gear (and obviously) in different rooms.

There's no guarantee that fans of R2R DACS would prefer the Lampizator just because you do!