Never put a mastering tool into the AUX-ways.
An exciter is always only used on the main out.
Meaning you have to use the master insert-points or directly between mixer and DAT, like you said.
Using an enhancer on a single sound is very dangerous, because enhancers and vitalizers add some frequenceys
(the even multiple harmonics of the ground-tones), which will cause the effected signal to sound different, mostly more clear and just more 'lively' than the rest. Which is the effect you want to have on the master signal.
Of course, you might wanna use it for weird effects on just some certain channels, but that's quite experimental.
And never switch it on before you are satisfied with the mix.
An exciter is always the last instance of the mixdown, but if you think you can leave it out, don't use it.
It mostly comes right after the limiter.
As said, it's the last thing you put on the mix, because everything that would come after it (Eq or whatever alters frequencys) would change the overtones/harmonics.
Which *might* sound odd on some stereos.