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?

I think you'll find quite a few fans here. I'm currently using an MHDT Lab Pagoda (PCM1704) which is 24/192 input/output capable. I can listen all day without fatigue and yet it's nuanced and detailed. I also have a dB Audio Labs Tranquility (TDA1543). This DAC can handle higher-res input but outputs at either 16/44.1 or 16/48. The sound quality is gorgeous nontheless and is another DAC I can listen to all day. For my headphone system I use a Schiit Bifrost Multibit (AD5547CRUZ) which is NOS only at 24/176.4 and 24/192 sample/bit rates. I guess you could say I'm a fan.
Thanks for the response.
I use a TDA1387 based DIY DAC. The chips costs pennies and is as simple as it gets as far as building DACS go. Performance for the price is quite astonishing.

This was one of the last NOS chips made, and features newer type of techonology compared to older ’’R2R’’ types which I dont have a hope of understanding, but some have said on DIY forums that this newer multibit technology is superior to older R2R in some ways due to not using internal resistors which inherently suffer from accuracy issues.
The drawback is only 16 bit support, luckily high sample rates are still supported which I think is the most important aspect of Hi Res.
Completely agree with you. I think those who haven't tried R2R DACs are really missing out on more natural sound. I would never go back.
My search ended when I purchased an Audio Note DAC2.1x Signature. It's NOS with absolutely no filters.

An 18Bit Analogue Devices chip is used (it's preferred over the 20 bit). In addition, there is a tube gain stage and tube rectification, and transformers are custom built in-house.
This dac accepts 44.1, 48, and  96KHz sample rates. 

It's been a transformational part of my audio rig.

Absolutely love my Schiit Yggdrasil A2. Has put digital listening on par with vinyl, still different but as enjoyable. Have also experienced great benefit from upgraded power cord for the Yggy.
There are a lot of factors , for example my Belcanto dac has
2 Femto clocks -that is Trillions, older dacs are in picoseconds. ,  much much better accuracy 
reading the digital signal , power supplies also Very critical 
seperate very low noise for analog as well as digital signals very 
important , and many other things . My New 2.7 dac sounds very much like the older types, smooth very musical and still very accurate. It is all about the implementation. Many companies 
do digital but not all are musically involving.
I also use a DIY TDA1387 DAC but my findings are rather different from yours @suix6. I've found analog filters improve the sound - best if the filter comes prior to the I/V stage. Could be I'm using different filters - LC (passive) ones.The improvement passive filtering brings is in the focus - somehow the instruments are rather ill-defined in the soundstage without a filter, they become more clearly delineated when the filter's included. Overall dynamics improves a fair amount too.
I'm still experimenting with I/V stages as these are perhaps the most critical part of a NOS DAC as regards subjective performance.
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.