Don't know about NOS dacs sounding "better", but the RME has a NOS filter which sounds the best to me out of the five that AKM 4490 provides. According to the RME manual this is the only filter that has essentially perfect impulse response.
A side effect of the NOS filter is that it slowly rolls off from 5k to 20k where it is down 3db when sampling at 44.1k. So it would be easy to attribute liking the sound of CDs through this filter to the roll off. However
@MC_RME provided the parameters to use with the parametric equalizer to flatten the frequency response again. Using this combination of NOS and PEQ you get perfect impulse response and flat frequency response. My CD play back now sounds extremely nice to me.
Actually impulse response with music is worse in NOS mode. On top of that EQ only increases the average amplitude to 'flat' but the treble above 10kHz is very poor in quality with extremely poor timing characteristics.
Is RME wrong ? No... indeed squarewave response and needle pulses are
produced (Note that I did not say
reproduced) quite admirably.
Unfortunately these well looking test signals do not exist in music nor in any recording. These are just test signals that exist so an engineer can evaluate filter behavior. The guys thinking that because these almost unlimited bandwidth (they are limited to about 100kHz or so) the DAC performs admirably are mistaken.
True, one can use squarewaves to test amplifiers and a nice squarewave response says a lot about the performance.
But here too it is merely a test signal and NOT something found in music.
In DACs these squarewaves and needle pulses are not supposed to look perfect. They are supposed to be 'rounded' yet reach the intended output level. Squarewaves are supposed to 'ring' as well as that is a side effect of the bandwidth being properly limited (so it complies to Shannon-Nyquist).
Have a closer look to an 'impulse' in music or a crescendo or sudden loud noise in any editing software.
You will see that such an 'impulse' spans multiple samples.
With a NOS DAC that impulse becomes 'jagged' and has extremely poor timing characteristics because it rises and drops at the wrong moment and stepped and too steep.
So while some people like it and feel impulse response has improved (because it is said to be so) in reality the actual signal is 'f'ed up, has lots of ultrasonic crap that might become problematic. In most cases it isn't problematic and the hearing is so crappy and bandwidth limited that it isn't even that audible when
average roll-off is 'compensated'. Only when 44.1 and 48kHz files are used. For higher sampling frequencies the EQ must be 'off'