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I have been reading for a while, the more i read the more confused i get. In fact i am having trouble getting my head around connecting it all up.

I have two cables each has a 3.5mm jack on one end and a pair of phono connectors on the other end (male, red and white)

The sound card has line in and a line out

I plug one of the cables into the line out using the 3.5mm jack. The other end of the cable, i plug the red bit into the red bit of the aux channel. I can now play a test tone from the computer through the amp and out of the speakers. I have a white plug spare.

The second cable, the 3.5mm plug goes into the line in on the soundcard. At the other end of that cable the red and white ends, i plug the red bit into the spl meter.

To create the loop that is required i took the white end of one cable and connected it to the white end of the other cable. This gives me horrendous feedback.

so starting again,

What i have done in REW so far, with a single cable running from the line out to the amp, red connector into aux, white connector not hanging free. No spl connected

I open it up
I select settings, i dont change anything
I press check levels
adjusting the volume on the amp does nothing, using the sweep i can up and down the reading on the screen, i set it to approx 75 on the screen spl meter. (sweep level 19)
click finish

at this point i go a bit pear shaped.