Unfiltered NOS DACs sound

I Have been experimenting with some NOS DAC chips and redbook material.
I very much like the NOS sound, and frankly think in terms of transparency it trumps any of the modern DS DACs Ive heard. The biggest drawback is THD and noise levels, on paper the modern chips out perform the older NOS chips and I think thats easy to hear but its superior ability to preserve of timing and spatial information in the music makes that problem relatively insignificant.

I have experimented with high quality software based upsampling, the conclusion is that any form of oversampling loses the ’’special’’ NOS sound.

Steep analogue filters did not affect transparency as much as software digital filtering, but the sound became dull and lacking vibrancy. I would choose the ’’processed’’ digitally filtered sound over this dull analogue filtered sound any day, but still NOS remains sounding the best.
I tried gentler (simpler circuit) analogue filters and more extended (filtering higher into the ultrasonic range), and the gains from the filters were never worth the filters influence on audible frequencies.

So as it stands the best sound I can achieve with these NOS chips is with a raw unfiltered PCM output.
The artifacts within unfiltered 44.1kHz PCM are very significant but ultrasonic, so it seems that the audible influence of this ultrasonic noise has on the system is not as siginificant as the means of removing it.... but this is probably not true in all systems.

What amazes me the most is that this is with redbook material... DS DACs and upsampling are supposed to be the more advanced methods of dealing with the limitations of 16 bit 44.1kHz data.
Unfiltered NOS naturally seems the most transparent and accurate way of experiencing Hi Res DXD and 192kHz material.

I know that unfiltered NOS has a sort of cult following, are there any other fans of it here?

Hello @abraxalito,  you are also a member of diyaudio? thank you for work with these DAC chips!

I have used RC and LC filters. For IV resistors and LM4562 Op amps. What you describe sounds like part of the filterless sound. The sound could be described as more wild and hazy compared to filtered. The calmer sound of filters would be the gains described above. 
Digital upsampling was the preferred choice for getting a more accurate detail and presentation.
Yes, I hang out on DIYA a lot. You're welcome, happy to share interesting results from these amazingly good value chips!

'Wild' sound could be it yeah, more 'refined' with the filter in place. I've tried running NOS and 2X OS, could barely hear the difference when the upsampling's run on my PC. Could be a nearby digital filter chip generates noise, we'll have to see. I plan to use a very low power CPU to do local upsampling.

As regards opamps, I tend to give them a pass nowadays. They need too much care and feeding on the supplies.
On my fith Metrum DAC and loving it.
Metrum Onyx. 
I use it for both my loudspeaker and headphone systems.