There is no standard industry definition for "dynamic" power. It means whatever the manufacturer wants it to mean. I discount all such references as marketing jargon of the useless variety.

If you think about it, if any part of the audio spectrum needs "continuous" power, it is in the bass region. I was at church the other day listening to their pipe organ. That beast (about 7,700 pipes - it's in an old cathedral) is capable of putting out some very sustained deep bass frequencies. If that's the music you're trying to reproduce with this sub and 50 watts isn't enough, then their "dynamic" rating is of no help whatsoever.

That said, the subwoofer's 50 watts may well be perfectly adequate depending on the volume level you're trying to achieve. The dynamic rating of 225 watts may only be present for impressing the susceptible. I'd recommend listening to the unit and see if it does what you want done.