Storm III or Stadium III for Martin Logan SL3's?

With the possible exception of the Studios and peculiar system matching, Rel subs are generally never "too big" for your room because as you move up the line you also get more speed/"quicker" bass. For best integration, you're not suppose to crank up the volume anyways, so it shouldn't be a matter of too much bass output. It's like the motor of treadmill. The bigger and more powerful the motor, the smoother and quieter it will be even though you never stress it.

The Stadium is a better choice. Have you thought about the ML depth or descent sub. I have the Descent with my SL3's in a smaller room than your's.The ML subs are better matched to their Electrostatic speakers. Both the Depth and Descent have won numerous awards. You can read them on their website.

I had the Stadium. Sold it and bought the Revel B15 - maybe the built-in EQ works especially well in my room, but it was a very significant improvement.

I have SL3'3...and the REL Stadium III...drive with Mac 402, Levinson CD and preamp, MIT Shotgun cables....sounds just beautiful.