I don't think the amp would have anything to do with efficiency at low to moderate volumes. In fact, you would be hard pressed to recognize any difference between that and an integrated receiver. The efficiency of the speaker is what is important.
The Axiom m80's are rated at 95dB's or so efficient "Inroom", which only requires "1" watt to achieve at 3ft distance. So an AMP with 200-300 watts of power will be no different.
The difference comes in at higher levels where you need enough headroom for dynamic peaks. For the majority of people, a decent 100watt or so receiver is more than adequate.
Not trying to detour you from getting an outboard amp, just making sure you understand that in most cases you will never use those extra watts.
