When comparing the 970 to the 950, there are a few distinct differences: 7.1 analog input rather than 5.1; transcoding of composite and s-video inputs to the component output; three component video inputs instead of two; DVI switching that is separate from the analog video switching; Pro Logic IIx instead of Pro Logic II with Cirrus Extra Surround; user-upgradeable firmware; a front input; quadruple crossovers instead of triple; individually adjustable distance settings for each speaker; and a headphone jack. The 950's analog bass management for the 5.1 input is carried over to the 970 with the addition of an option for digital bass management of that input. If any of those features appeal to you, the 970 might be a good upgrade from the 950.
When comparing the 970 to the 1050, the list of differences is even longer: Dolby Digital EX; DTS ES; DTS NEO:6; Pro Logic IIx instead of plain old Pro Logic; quadruple adjustable crossovers instead of a single global adjustable crossover; component video switching with transcoding (instead of no component switching at all) and DVI switching; bass management for the 7.1 input; a stereo bypass mode for two-channel analog inputs; and all the things mentioned above. The 970's sound quality is going to be better, as I recall the move from 1050 to 950 was a nice upgrade in that regard and the 970 should at least match the 950.
The 990 is going to offer a little better sound quality than the 970, some more adjustments (editable input names, assignable component and DVI inputs), an automatic setup routine for speaker distances and levels, and a few extra inputs. It also omits the analog bass management for the 7.1 analog input, although it provides the digital bass management (with quadruple crossovers) for that input. It also offers a phono input.