Purifi Class D: Junk?

You have the wrong buffer. Call Warren and talk.

Yes, to echo the above. The input board you have in your VTV is the Hypex eval board that was originally for the NC500 module. Purifi has also made an input buffer board (both stereo and mono) that uses different regulators, op amps, etc. Warren at VTV is now going to sell those, as well. However, the Purifi one won’t sound as good as the VTV custom input board with discrete op amps......but, of course, there is lots of other things you can do to improve the sound. What you have is the least good sounding Purifi version that anyone can buy.

What you have:
https://www.hypex.nl/product/nc500-evaluation-board/51

What Purifi offers and now VTV sells now too. Check out Eval 1 and 2:
https://purifi-audio.com/vare-kategori/transducers/

What VTV sells as an upgrade that is better than the above the two:
https://vtvamplifier.com/product/vtv-amplifier-custom-input-buffer-hypex-nc500-nc1200-nc2k-purifi/

Even a dual mono VTV with the VTV custom input buffer boards using the best buffers will not give the best sound.  There is much more to be gotten from tweaking everywhere.  You cannot do this at this price point.   It is all plug and play at the $1500/$2000 level.

What Purifi offers and now VTV sells now too. Check out Eval 1 and 2:
https://purifi-audio.com/vare-kategori/transducers/

I do not believe that VTV offers the Eval 1 and 2 buffers, and nor have I seen any other vendors offer amplifiers with Purifi Eval buffers. On Purifi's order pages, they do not sell the Eval boards separately (only in kits combined with 1ET400A modules).

This thread is a great example of why I chose many years ago to avoid DIY. How many ways does one wish to swim in the quicksand of slippery standards of sound quality, with no assurance of a result with actual great sound? I'll give the manufacturers their due; it's a lot better than wasting my life/time/money building a mediocre components that are not worth hearing. 

It didn't take genius to see that the amp, speaker, etc. kits all have fundamental caveats. You don't get superb on the cheap. The odds you will make huge mistakes are very high, as evidenced in the thread here.
So, you spend your days slaving over a build that ends up an also ran. Not my idea of how I want to spend my audiophile life. Maybe if I was thrilled to look at circuits, it would be fun. But, what fun is it when the sound is so-so? I'm not into this to see how good I can make cheap audio. That's all but assurance that one plays in the shallower end of the pool. 

Kudos to those who want to play in DIY for the learning, for the fun of it. But, that's not me. I can't stand compromised sound, and I won't accept it long term. If it can't perform at a high level, then pride of build/DIY doesn't matter. Same with vintage; can't take it, as the sound quality is just not there typically. It's cheaper, it's nostalgic, but it's nowhere near great. 

What's interesting here, too, is how many times people have tried to tell the OP that he's missing things, but the self-confidence and skepticism is off the charts. No wonder he'll not build a superior amp. Having used discrete opamps rolling into the EE Minimax DACS, I know firsthand how they can contour everything related to the sound, not only tonality. The OP is hard core skeptical, so it's not surprising to me he can't get the amp to work great. This should be a cautionary tale for those who tend to "think" what should or should not work. This is a perfect example of how theory on the cheap doesn't build great components - or systems. 

This site has become rife with people like this; really proud of their objectivist perspective, and declarative of what should or should not work. Only one problem; it doesn't build superior systems, it builds average systems, nothing worth writing home about. 

It reminds me of some of the industry members, manufacturers, with the same attitude, very skeptical of anything other than what they consider to be supported by theory. They tend to build to spec, with the cheapest parts that should do the job competently. These are not the best components on the market. They serve a purpose for the lower cost audiophile, but there is significant compromise to them sonically. These components over many iterations of systems never rise to the top; they never best superior gear with higher quality build and methods. 

@douglas_schroeder - I don't think you read this thread very carefully. This isn't about DIY (although I don't personally share your skepticism about DIY). The OP purchased a commercial product using the Purifi module, a basic SMPS and cheap input buffer, and is blaming the Purifi module for its disappointing sound quality instead of the surrounding parts and overall design. 

That's like saying all DACs that use the ESS9038 are crap because a $79 AliExpress special sounded mediocre.