George, the residual **is** the switching frequency. So this bit
so by the time the residual reaches 20khz what’s left with the 1.5mhz is down nearly 3 lower in amplitude and has less phase shift in the audio band than that of normal 600khz switching frequency.doesn't make sense as written. I think what you are trying to say is that with a switching frequency 3x higher, the residual will be reduced (given that the filter in question is the same whether the amp is switching at 600KHz or 1.5MHz). Its helpful to express this in db; assuming a 6db slope the residual will be down an additional 9 or so db. Which is nice!
The 20KHz part has nothing to do with it. Whether you measure at 20KHz as opposed to 20Hz (if you change the timebase on the oscilloscope if you are observing either frequency as a test signal) the residual will be the same amplitude.