[QUOTE=LiloLee] Valves won't suffer from this problem, but am interested why digital ones also appear to be OK (seen this else where as well as with Michael and Ian)[/QUOTE] The output of a switching amp is low-pass LC filtered. The switching noise is reasonably far out of band, so a simple two component filter can be used. This filter will equally have an effect on any ultrasonic aliasing components that might be present in the input signal.