Some companies have tight tolerances (JL fgor instance) and the power will always be just a few watts away from advertised (up or down).

Other companies (maybe most?) will have power, like this amp, that can be around or over 20% difference in power... Some birth sheets will be righ on rated, others will be a lot higher. That's simply them knowing there tolerances are to wide and so I imagine they rate them close to the lowest they can be.

IMO it's that they don't have tight enough quality control. But maybe I am wrong.

 

OP - If you're happy with the amp...that's all that matters.

 

The JL amps also use a tightly regulated power supply where the MTX use an unregulated supply. With tight regulation the output section of the amp gets the same power regardless of the voltage at the input terminals. With an unregulated supply, the more voltage you supply(to a point), the more power for the output section and thus the more output power you get at the terminals. The amp will be rated for 12volt input but the birth sheet is often measured with 13-14.4 volts, accounting for the extra power output.

One isn't better than the other, just different.

 

Well indeed if they aren't measured the same then it's a moot point and it's not under rated in that case.