Originally Posted by W I G
Can you explain the sub part more?
Subs (well, any speaker) by design are dirty. They make a lot of heat which throws off tolerances, they have larger radiating surfaces that can bend and flex, they're phase nightmares, etc..etc..
Most of the "distortion" you will hear from a speaker comes in the form of higher order harmonics and mechanical noise (even though it's not strictly "distortion", it does color the sound). Probably every amplifier on the market will have a low enough THD figure to be unnoticeable audible. That is assuming they are run within their tolerances. Notice that the amp you listed in the original post was rated at 4 ohm and at 40 watts to get the THD.