Rhis might be a stupid question but im just wondering. Lets say you have two of each:
ef/el premium AMP and CD Stacker

Can you find out which of them is a better one then the other? they all run but is there anyway of doing some kind of a diagnostic on em?? I presume to check the stacker would be putting in cds and playing them and checking if they dont skip etc etc.

As i said its a stupid question but any info will help.

AFAIK they are identical.

u mean like out put performance?? or if they read and play cd's?? connect it up to 12 volts, putr a cd in play the cd with out and speakers
and test the volatage its putting out from the speaker wires, but they should be all the same.

Can you say mate why you are wanting to do this? as in are you looking to upgrade one of the components etc?

About the only way a non-tech could get an idea of whether the CD stacker is working reasonably well is just by using each function and seeing how fast it is done compared to the other CD stacker.
For example, watch how quickly it selects a disc and reads the TOC. Then hit track fwd / rwd and see how long it takes to move to the next / previous music track. You can hold down the scan button for the length of a full song and watch the display and listen to the music track as it scans - if the laser's tracking is down then the music track being scanned will skip or hang and will be quite obvious.
If you're a tech then I'd advise resoldering the main PCB as these factory CD stackers tend to develop dry & cracked joints over time and operation may become intermittent. Just be mindful of not bridging any tracks on the main PCB!