Just use your audyssey mic, connect it directly to mic port on your PC, use HDMI port (or whatever) on PC to generate tones/sweeps from REW (free software). I think you can calibrate SPL meter in REW to around 70Db (by using test tone generator on receiver) just to put something in. Then you can change setting on receiver or sub (or unplug speakers if you only want to see sub, just don't short them

), rerun sweeps and see how it will affect FR. You not going to be perfectly accurate, but relative sweeps will give you exact picture of what the setting are doing...
you can find detailed info about using REW on
Home Theater Forum - Home Theater Systems - HomeTheaterShack, just skip the parts that tell you to buy extra hardware.
Completely free solution...