[eljay]

Would there be any point in adding the sub for stereo?

Simple answer: Try with and without and go with the set-up that provides the best response.

Longer answer: Although the RC-50s go to 33Hz +/- 3dB and the sub goes to 32Hz (presumably also +/- 3dB), you may get better response down to those lower frequencies from the sub since it has a larger driver and a dedicated 150W amp, and its sole purpose is to produce bass.

Based on what I read in the owner's manual, set all your speakers to "Small" and set the subwoofer to "LFE". The subwoofer will handle all frequencies below the crossover point (say, 60Hz for the mains and 80Hz for the rest) as well as the LFE.

 

[eljay]

FWIW, I tried a number of different combinations and settings w/ my current set-up before settling on the following:
- Stereo - "Pure Direct" mode (main speakers only, no audio processing).
- HT - "Auto" mode (occasionally matrixed to 7.1), Audyssey's "Audyssey" setting (i.e., full EQ), all speakers set to "Small" and crossover set to 60Hz.

YMMV.