Can I used the parasound a51 with B&W 703 speakers

Yes of course! Under powering speakers does more common damage than over powering.

An lets say, 100w per channel amp on speakers rated at 200w
will die [distort] trying to "push" em

Take 400w amp same speakers, any distortion will be on speakers end. Speakers WILL let you know when you're driving em too hard.

Have Monster mpa 2250's bridged 600w per channel driving my mains. No problem!!!! Wharfedale EVO 40's

Also B&W rates their speakers on un cliped program material
This is what they mean. Clean power and plenty of it!!!

Hope this helps
Bruce