The difference is in the power supply and the impedance dip of the speakers. As I have read in many magazines: Sensitivity is NOT= to sound quality.
Even with a 90db sensitivity, if the speakers dip to 4 ohms or below, more current is needed (where do you think they get the juice to power all those woofers & tweeters?). A receiver will not be up to the task of powering it, as they werent really built to handle 4 ohm loads well (except for pricey THX ultra models, they are big & heavy too). See HT buyer's guide mags, 90% of receivers do not have a 4 ohm rating.
Why? because of the limited power supply and heat dissipation.
But why can 2 channel integrateds handle them well when a "higher" power receiver cannot? Its in the power supply. Notice that torroidal power supplies are available even with 15k below integrateds but it will cost you 100+k flagship models for a 7.1 receiver with torroidal power supply. And they usually need 2 people to carry.
Also imagine the
heat dissipation needed when a receiver heats up to power 4 ohm loads. I have been using receiver-power amp combinations for some time now and ALL 4 receivers I used as pure pre/pro heat up like mad
just doing its processing job alone (no amplifier used yet). Imagine the heat a receiver will generate doing its pre/pro job PLUS powering 7 speakers, specially if they are 4 ohms or dip to 4ohms (w/c most speakers do btw)!
Thats why PRE OUTS were invented!
Just my 2 cents.