Have you also tried Pro Logic IIx with those stereo channels? If you still prefer 5 Stereo, then I'd suggest setting the STB on one of those channels, going into the 990's menu, going to the setup screen for that input, and setting the default surround mode to 5 Stereo.

As for good search terms, I'd probably have to start with "5 Stereo" because I'm not sure what might help filter more precisely than that.