Steady state frequency response is more often than not going to show up flat on a scope. But music is dynamic and there is when the frequency response can be far from flat. Excessive highs is quite common in amplified outputs stages in a DAC that then have to use a Sallen-Key or MFB anti aliasing filter. Those kind of filters can easily produce a harsh type of sound if the output stage has been set up for maximum detail and the output uses opamps. FETs transistors and valves were the most obvious cures to the problem, but unamplified outputs are now finding their way into the market with the introduction of DAC chips from several manufacturers.