Lifted from another forum cause I can't be bothered to type an essay. 
I'll take a stab. MOSFETs are a type of transistor - the device that generates the power to the speaker. The other types of devices that do the same thing are tubes and bipolar transistors. Each have their pros and cons. they are usually run in a class A or class AB types of amplifiers. The class is they way the transistors/tubes used to generate the power.
Class D is a different animal altogether. A and AB are analog amps - sine wave in = sine wave out only bigger. A class D amp, also known as a digital amp, turns the sine wave in put to a series of pulses whose width correspond to the amplitude of the pulse then the pulses are amplified and run through a low pass filter to get rid of all the nasty edges and turn it into a sine wave only bigger. The benefit is much higher efficiency so the power supply can be smaller and the heat generated is less, which theoretically will translate in smaller and/or cheaper amps, such as the EA and Clarus models (smaller but *not* cheaper). Class A and AB amps are easier to design, and to most people sound better (always subjective), and usually don't have noise and RF issues like class D amps can.
That's the basics, hope it was somewhat understandable.
On a side note, digital power supplies and all-ditigal amps are different too. The digital power supply is again smaller and more efficient. A digital amp, say Line6 pod, is an amplifier that turns the input from the instrument into a digital signal and does all of the effects and tone shaping digitally (in a computer basically) and then turns it back into an analog signal at the output of the amp.
And a second explanation again from another forum.
Typically Class-D will be smaller, lighter, and more efficient than an equivalent standard "analog" power amplifier, whether the analog amp uses mosfets or bipolar transistors, or tubes.
Good Class-D is very good fidelity-wise. The best class-D is as good or better than the top audiophile Class-A, class-AB, Class-H etc amplifiers.
A Mosfet used in the linear mode as a traditional "class AB" power amp is, well, an analog amplifier.
In general, they are larger, hotter, heavier, and less efficient. At worst, well over half the input power simply becomes heat that must be dissipated, so the efficiency can be well under 50%. So they require a larger transformer (or other power supply), large heatsinks, usually fans, etc.
Most, if not all, class-D amplifiers also use MosFets*. I don't know of any current one that does not.
A class-D amplifier produces output pulses instead of a continuous waveform, and uses an output "reconstruction filter" to re-create the analog waveform from the pulses. Since the pulses occur at rates in the range of several hundred kHz to over 1 mHz, to your ear it is continuous.
For various technical reasons, it is more efficient to do that, it wastes less power, creates less heat, etc. Efficiency can easily be well over 90%.
Transformers or other power supplies can be smaller, and heatsinks smaller, fans smaller if used at all, etc. Less heavy stuff to carry.
While class-G and class-H multi-level power supply type amplifiers can be more efficient than class-AB, the efficiency is still not that of class-D, and the units are considerably more complex.
My power supply was 0 awg Hdi.
I.C.E Reccomendations 
What kind of cable was feeding the power amps? 
Linear Mode
