CEA-2006 compliant, nuff said.
CEA has a list of standards that require the amplifier, wiring, speakers, headunits, etc. to meet certain standards in order to advertise that it does X, Y, and/or Z.
The JBL amplifier, and most large name brands are CEA compliant. Knowing it's CEA compliant means you know the power it's advertising to produce, it does produce.
Besides that, make sure the amplifier is tuned correctly, and if nothing else in the audio system has changed, then you can probably contribute it to the amplifier.
Not to mention seeing a singular 40A fuse on a class A/B amp that supposedly produces 600 RMS tells me it doesn't even come close to the 600W RMS.
For the most part you ALWAYS get what you pay for in one way or another.