Compression
As for the Compressor that you want to buy...that shouldn't be used through AUX, as it would be wasted. It needs to somehow be directly inline with the source, whether vocals or an instrument.
Think of Aux send and returns verse Insert and inline connections this way:
Aux Send and Return: Parallel signals
Both the original signal and the effected original signal are
added together at the end....or summed up.
Insert or inline: Series signals
Source signal is introduced to the effect. The output is now
just the effected source, rather than the effected
source+original source. They can't be separated at the
end...
Typically, effects, such as delays and Reverbs, are run through auxilliary sends and returns, where as compression, limiting, feedback destroyers, etc. must be run inline through Inserts or directly on the cable from individual sound source (such as a distortion stompbox).
If you used a compressor/limiter through aux sends and returns, the result would be a mix of the controlled output, and the original output that is in need of control...so if somebody is performing vocally, and they tend to sing quietly, then peak...you might want to tighten up their range with some compression and limiting, but you've defeated this if you tried to do it through Aux sends and returns.
Hopefully somewhere here I used the right words to steer you in the right direction!
