Hello John,
Tried for some time yesterday with the audio settings of the imac. All it says is that line out is not supported. I had faced this problem about a year back when I tried to get the TV audio signals in the imac and use it to feed a small 5.1 system.((Although the itunes, etc worked well.)
I phoned apple for support.
Reply: The line in works only if used with a software like "Sound Studio"
I installed sound studio and it worked but I could not use any controls of itunes eg. equalizer.
Sound Studio has a tab which you can tick and it sends the line in signal to line out straight through.
So yesterday as I said I tried using REW with sound studio support.
It works in as much as accepting the spl meter signals and spl calibration.
The rest does not work.ie. The sound card cant be calibrated.(It shows a very large dip below 100 Hertz.) The left channel cant be looped back.
The sweep works with all its function filters etc. However, accuracy is questionable.
Secondly I decided to purchase the DEQ2496 to use with REW. I found a calculator to convert BW to Q. Here is the link.
http://www.sengpielaudio.com/calculator-bandwidth.htm
What do you think about it John?