I was browsing Crutchfield for an amp for my speakers and noticed the Alpine class D amps have THD <0.1 %, while the other class AB amps are all at 1%. Does this result in cleaner sound or is it just because of the amp designs that they can post those numbers?

Here is a link to one:

http://www.crutchfield.com/p_500MRXF...82#details-tab

 

n short, Class D amps are more efficient but are only good for low frequencies (just for subs). other amps are generally less efficient but can be used full range (ie. for speakers too). what this means is that, if you had a 500 watt Class D amp and for comparison a 500 watt Class A/B amp (the most common I believe), the Class D amp would have less of a current draw on your battery because it requires less to make the same amunt of output.
However, Class D amps can't be used on highs because of the way they work. they only make square waves because of the technology involved, so they will make your highs sound lifeless and ****ty. Class A/B amps are full variable and can capture subtle nuances better.
this is why most high power sub amplifiers (with a few exceptions) are Class D, because that way you can get gigantic output from a 12 volt amplifier that otherwise would be very expensive in terms of final cost and power to get out of any other amp... for instance, a 3000 watt Earthquake Class D is around $600 brand new while a 2000 watt US Amps Class A/B is around $600 used.

 

90% of what you said is NOT true. The only thing that I can see that you are correct about is that class d is more efficient.

 

I ask because I'm torn between a Rockford Fosgate Punch 4-channel class AB and an Alpine MRX 4 or 5-channel digital class D.

 

OP, just get whatever fits your budget. The 5 channel would be nice to power your interior speakers and a sub if you decide to go that route.