Here are a couple things to try:

• With coaxial/optical output, try a calibration disc with Dolby Digital or DTS test tones and see if the subwoofer test tone works.
• With the multichannel analog output, try the AIX test disc's subwoofer test tone.

If the first doesn't work, that would suggest the wrong setting for the BDP-83's digital output or an issue at the 990. If the second doesn't work, it would suggest a setting mistake or issue at the BDP-83.