I will try my best to answer these questions but others are far more qualified.
An amp can be used to drive set ups that demand a lot of current. I know that some out there feel the more power the better sound but I find that one hard to swallow. If you have a speaker set up that eats power to run them correctly (axioms are not one of those brands) then you may want to get an amp. Also, like you mentioned, if your listening levels exceed those of normal human beings then you may want an amp. If you want an amp, to ty out, get a used on at audiogon or ebay. I don't know the name of it but there is a canadian audio trader as well that you can use to save yourself on the import.
The signs that you need an amp are if when you turn your reciever WAY up it is still not loud enough.
Just my 2 cents.
