I don't know about MTX, but JBL has been in the amp business for years. Car audio is relatively new, but they have been rock solid in pro audio for decades. I have a JBL amp and am perfectly happy with the performance.
There´s nothing to say about the performance altho they could have putted more money in their power supply´s on the lower models but the quality of sound is a other topic...
MTX as a company is very reliable and has a pretty rock solid productline altho I find them to expense for the power you get but thats just a personal choice.
Comparing JBL and MTX is a no no ...
MTX = Power and quality
JBL = quality on getinclass and power
owye.. subsonic filter is another diff and also the impedance detector on the mtx is a nice subsaver