Bombaywalla, Linear power supplies have regulation in preamps, DACs etc. but MOST of power amps are unregulated because of the amount of heat dissipated in power supply (and loss of efficiency).
AFAIK older REF1000 and newer REF1000M use the same module, but REF1000M has additional separate power factor correction module.
Look at fig. 2 showing how PWM signal can be obtained by just using linear ramp. Icepower modulator uses sinewave, has two feedbacks etc. - but only for improvements.
http://en.wikipedia.org/wiki/Pulse-width_modulationOne limiting factor in class D is bandwidth. Latest Rowland class AB amps have bandwidth of 350kHz while newest class D model 925 50kHz. My small 102 extends to 60kHz. It is limited by the speed of Mosfets currently available especially high current devices, but it is improving every single year. We definitely cannot hear 350kHz but wide bandwidth prevents phase shift thus preserving harmonics within audible band. My amplifier with bandwidth of 60kHz shifts phase of the 20kHz signal by 20 degree.
The difference in sound, with class D, I observed is that sound is more immediate, faster but at the same time there is less of brightness in high frequencies. Cymbals sound less "splashy" but more brassy with more body. Perhaps it is related to lower TIM distortions or it is just the nature of the beast. It is definitely different but I like it.