The proper range is not +/- 3.5 dB, but +/- 12 dB for the satellites and -15/ +12 dB for the subwoofer on your Onkyo 717. That's a whole lotta difference.
Umm, ...actually the Onkyo 717 is another entry level receiver as regards the 2EQ version of Audyssey which does not EQ the frequency response of the subwoofer channel, only sets distances and speaker levels. In case your Onkyo turns out to be defective why not upgrade to a model that has at least MultEQ on board (with sub channel EQ'ing), as far as funds allow you this upgrade?
Not batpig, but it surely is the cable from AVR to Subwoofer.
Finally, as a last resort, did you try a microprocessor reset?