well 93 db would mean to double the power required, which would be putting a rather large load on the amp. you likely wouldn't notice it unless for extended listening, onkyos are really great at handling such circumstances and if the amp was struggling it would work out a solution for instances such as peaks. this is assuming the amp was struggling, which may or may not be the case, but i think 50-60 watts could be a bit of a struggle and this possibly would be when distortion could begin to become noticeable.
they do rate the onkyo at THD of .08 at half power, not sure what half-power means without showing us total power and ohms/channels/etc but I assume its taking that from 100w (which is what many avr's are marketed as).

i don't think this link pertains to your avr, but is similar and may provide some insight: http://www.soundandvisionmag.com/art...09-av-receiver
the last paragraph especially