Yeah, I agree with MACCA350. You need to test the receiver by itself to get an accurate read on what is going on. I'm surprised that the analog input (or analog output) would have such a poor low end response.
Hook up a cable to the REW line-in and another cable to the REW line-out. Then connect those two cables together to create a loopback from line-in to line-out just as if you were doing a soundcard test. Now do a REW response measurement on that loop and ensure that it is perfectly flat from 2Hz to 200Hz. It should be, because the soundcard cal file should force it to be so when you are taking a measurement of a cable.
Now insert your receiver into the loopback and take an REW measurement. Since we know that the cable by itself is perfectly flat, then the response we get by simply measuring the receiver will be an accurate reading of its response. All you need to do is adjust the receivers volume control to get the REW levels set up. Use the CD or AUX input and the subwoofer output of the reciever.
I did the response of my processor and get the result below. You can see it tracks the REW crossover target and then drops off a little at the lowend. I did both the analog bypass plus the analog path with the ADC/DAC engaged. You can see the analog bypass doesn't provide any bass management, but gives a bit better low end response.
A-cable vs sp2 SUB OUT adc-dac.jpg A-cable vs sp2 SUB OUT in bypass.jpg
brucek