SACD cannot pass over coaxial or toslink. For the case of stereo SACD's, the problem is mainly copy protection (unlike DVD-Audio, which allows a downmixed 48khz/24bit PCM stereo output over coaxial and optical, SACD elects to give you absolutely nothing). Once you get to multichannel SACD's there's not enough bandwidth anyway (it's a lossless multichannel signal, so it won't fit).
To play SACD's through the 990 and hear anything, you need an analog connection (which for multichannel SACD's means the 7.1 input). If we look instead at the 997, then you have the option of HDMI - which can handle SACD's output as multichannel PCM, and which may also handle it as a native DSD bitstream (although that also requires a player that will do DSD output, which a relatively small number of players will do, and we don't have official word on whether or not the 997 will accept a DSD bitstream).
In your case, nfaguys, if you connect the 981HD to your 990 via multichannel analog, you can play SACD's. If you connected the 981HD to a 997 via HDMI, you could get SACD's as multichannel PCM. The 981HD won't output DSD.