Which player are you looking at? I looked up the Toshiba HD-XA1 HD DVD player and it has optical output, digital cable output, HDMI output and the 5.1 output you mentioned. There are several ways you could hook this up to the 990. I think one option might include getting a HDMI to DVI cable for the HD video and running your audio through one of the above mentioned formats.

Toshiba HD-XA1 Rear View
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Outlaw 990/7700
SVS 20-39 PC +
SVS MTS-01 towers, MCS-01 center, MBS-01 surrounds
Behringer A500
Samsung PN58A650, DirecTV HR-20 700
Sony CDP-545, Phillips CDR 765
Oppo BDP-93, Panasonic DMR E515
Technics SL-DL5
Squeezebox 3
Remote Harmony One