"The Grado's are in fact normally less sensitive according to their specification but they are a decade lower in impedance so draw considerably more power. As power density increases so will noise density. Usually you'd turn the volume down and everything would sound the same. However, the Grado transducers are much smaller than the HD650 (and similar Sennheiser's) and constitute a "point-source". In listening to vinyl (which few do these days) the vinyl noise is heightened possibly because the "point-source" is aiming it all much more effectively into our ears. Therefore any hiss will also sound more apparent. The old valve gear was quite hissy and it's due to there being much less negative feedback than solid state. Less negative feedback equates to more hiss. Likewise in mimicking the valve the ultra linear will be more hissy than its SRGII counterpart - but when the music's playing the hiss should be drowned out."