NOS DAC Warmth

I think it is dependent on what generation of NOS you are talking about. The older chips do tend to lose some detail, no matter what the rest of the system is like. I used to sell a NOS DAC based on an older chip.

Modern custom designs are much faster and retrieve all of the details.

Jitter will be a factor too, so the interface used and the quality of the master clock will affect this. Jitter will obscure detail if it is too high. Customers of mine that get lower jitter sources often say that it is "smoother". Not the way I would describe it, but sometimes eliminating the edginess or "fill" of jitter can be interpreted as smoothness.

Another thing to consider is that even an oversampling DAC can sound smooth if the digital filter effects are minimized.  DACs with selectable filter cutoffs enable this.

Steve N.

Empirical Audio

Adding distortion, such as poorly designed tube devices, can add warmth. This is a Band-Aid at best, masking distortion by adding more distortion of a different type. Kind of like using cables as tone controls.

It is better to rid the system of distortion that causes harshness, such as jitter from digital devices and poorly designed preamps that add distortion and compression for example.

Not all tubes add warmth BTW.  Well-designed tube circuits are every bit as fast and clear as SS devices.  This is mostly a function of the power subsystem and the tube selection.

Steve N.

Empirical Audio

Each to his own.  I prefer to have my cake and eat it too:

https://www.audiostream.com/content/empirical-audio-overdrive-sx-ethernet-dacpre

This sounds like the midrange of tubes, but it's SS.