100% MOST CERTAINLY!!!!!!!! NEVER run the "S" without a load on both outputs - EVER!!!!!
Secondly, with regards to your questions about volume - spread and SPL could be louder because the head can power 2 separate cabinets. The spread of these 2 cabinets can create a bigger sound, and pending on how they're matched, they may seem louder than one cabinet powered by one head. But when the math is done, instead of 100W going into 1 load, it's 50W going into 2 loads.
Peace,
V.
PS - the key bonus with the "S" model is true stereo panning effects and the fact you can spread your sound apart. Nice.