Yesterday I visited my friend who has inherited my first round of purchases - a Marantz PM7005 integrated amp rated at 60WPC and a pair of Focal 716V speakers rated at 91.5db@8ohms. He has set these up in a fairly large family room open to his kitchen. The approximate dimensions are 18' deep x 24 wide 'x 12'high. His main listeneing position is about 10' from the speakers, which are 2' from the back wall. There is about 6' between his couch and the rear wall which is mostly glass onto his patio. ( BTW this is a beautiful New England farmhouse fom the 1700's that he personally restored over 4 years, including building over 400' of handlaid stone walls around the perimeter of the property. Building stone walls is his hobby - he is certainly not afraid of a little hard work)

While discussing the power needed to drive the speakers at a level suitable for both personal listening and while entertaining, I suggested the following rough math:

To deliver 91.5db @1meter takes 1w of power. To increase by 3db requires doubling of power and to maintain same listening level 1 meter farther away also requires doubling of power. Therefore to listen at 97.5 db (likely as loud as you would ever want for a party) and to maintain that at 3 meters from speakers ( ie midpoint of room and his typical listening position)power must double 4x or to 16w. To maintain sound level at far side of room would require another 3x doubling or 128w. If he wants to maintain listening at 94.5db he needs 64w. Less power than that and he will experience clipping. Given the Marantz is reported to be conservatively rated, it may be sufficient for most listening, but undersized for " filling the room".

So is my rough math completely misguided for estimating amp size? And if this is generally ok, how do you account for dips in speaker efficiency ( I believe mmany Focals have dips down to 3ohms at certain hz which place additional demands on power)?