I've been trying out different preamps and amps with my Sierra 2's and found the PrimaLuna integrated with EL34 tubes to sounds truly amazing. The sound was better than my ATI amp with the Rotel 1590 or Parasound P5. That being said the PrimaLuna only has about 36 WPC and I noticed the specs for the Sierra 2 state 45 WPC minimum.

What should I watch out for by using the lower power amp?