Well... this may seem a bit off-the-wall, but believe me, I wouldn't suggest it, if people hadn't DONE it, and it WORKED:
What about taking one of the un-used inputs on the 9090 (aux in, etc... anything but a phono in would work), and assigning it to the pre-outs for the left and right front channels of your HT receiver? Then, just disconnect the speaker wires for left and right front from the HT receiver completely... use the Sansui AS the amp for the front left and right speakers, EVEN in HT mode.
All you really have to do, is to determine what "reference" volume control level, on the Sansui, that you would need to set, to get the gain right to match the rest of the home theater speakers. Once you set the right input and set the volume control to the right place, then the Sansui would just be acting as a "glorified outboard power amp" for the HT receiver.
I've not met a home theater receiver, YET, that didn't perform BETTER without having to drive ALL the channels at the same time... if you're only running 3 out of 5 amp channels, the power supply will do a MUCH BETTER job of supplying those, due to the decreased total load. And I bet, the Sansui amp section, is considerably more potent than the HT receiver. The system will probably be more dynamic and probably will generally just sound better...
I've even done this with "exotic" 2-channel setups, to "expand" them to home-theater... stuff like Aragon and Conrad-Johnson stuff, and the like. Exactly like described above... just using the 2-channel rig as an outboard amp and pair of left and right front speakers, for the HT rig, and running the center and the rest directly from the receiver. Other than having to remember to set the 2-channel rig to the right settings (volume and input), it's no harder than running the HT rig by itself, once you get everything plugged in and figure out where your volume control need to be set, to match gains...
Regards,
Gordon.