Keith and Paul.
NOS DACs from the figures I have seen do not notably roll off the top end.
They do process signals in a different way to over-sampling and filtering DACs.
Tube DACs Paul do add some distortion. However, as always in hi-fi it is not whether something adds distortion, but rather the type of distortion that matters. Simple overall distortion is one thing. Second or third harmonic distortion is something entirely different to the ears of the listener, especially if present at extremely small levels.
I'm perfectly happy with my tube pre and my non-filtering DAC adding some distortion. Whether it is significant enough to affect the output at audible frequencies is another matter entirely. And if it does, whether is adds or subtracts from the listening experience is again another matter.
Keith. Apologies, but everything you sell distorts the signal from the original performance to some degree. This is by no means always a bad thing and often the distortion in a reproduced signal actually benefits by adding some ambience to the eventual result.
Live with it.
None of this is ever the live performance. Anything that manages to provide some of the 'feel' of the original is a result. Distorted or not.