The quality of the watts is more important than the number of them and 50 watts from a decent quality amp should be plenty for the Ushers. My daughter has the S-520's driven by a 40 watt Pioneer Elite integrated and power isn't an issue even in her fairly large living room.
I agree with Loomisjohnson that given the small number of choices and your limited budget you may be better off with an integrated or receiver from a company like Cambridge Audio, NAD or Rotel coupled with an inexpensive external DAC.
I agree with Loomisjohnson that given the small number of choices and your limited budget you may be better off with an integrated or receiver from a company like Cambridge Audio, NAD or Rotel coupled with an inexpensive external DAC.