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

Hi,None, better to have more power on hand than what you will need rather than under power your speakers. Now that doe'nt mean go nuts and turn the volume up past the point where your speakers start to sound strained or distorted. Keep in mind the A51 is a beast of an amp, I have the A21 which is 250x2 and my speakers are rated to handle 400 watts and I have pushed my system hard at times but have not reached the limit of amp or speaker! chicken I guess! Should sound great enjoy!