Jack, I was able to just lower everything down and make it match at 75db. Thanks.
I'll go through my method. If anyone's still with me on this, please find any faults if you have the time.
#1. Fronts set to 'Large' on Denon.
#2. Optical cable from computer to Denon.
#3. RCA from 'front left' into stereo 'Y' to computer input.
#4. RCA from SPL meter into other side of 'Y'.
#5. Under soundcard settings -
a. I select 'use left channel for calibration reference?
b. hit 'check levels' and adjust Denon volume so meter reads 75db
#6. Under mic/meter settings -
a. Select 'C' weighted ?????
b. for calibration, load the correct radioshack meter file (33-2055)
#7. Main menu - hit 'measure' go from 10-200 hz
#8. Save that curve and load it into the sound card calibration setting?
#9. Uncheck 'C weight' and retest. This should give a flat line.
#10. Unhook loop, deselect 'use left channel for calibration', then run again to find the final outcome???
If that's correct, here is what I have:
The blue line was the sound card calibration. Looks odd to me.
The green line was to test to see if it worked. It's flat like it should be, so maybe it's okay?
The red line is the final result. My subs are crossed over at 90 I believe.
At no time did I change the SPL meter, it was left at the 70 'notch' on the wheel the whole time. But when I was done, I decided to change the actual meter's knob up to 90, then measured again with everything else the same. The graph looked different, but why should it?