If you want to hit TRUE reference with your setup, an external amp is almost always neccessary. Go to this site and plug in your parameters:
http://myhometheater.homestead.com/splcalculator.html#anchor_13115For me, on a good day my Onkyo is 'supposed' to output 100 watts per channel. In reality, it closer to 75 (assuming all channels driven). But let's take the 100 watts as the best case. I sit 8 feet from my M60s. With my setup, the PEAK SPL I can get is about 100db. That is 5 dbs from reference. To get 103db, I would have to have 200 watts. To get 106, I would need 400 watts (double the power for each 3db gain). I do not know of any AVRs that will output 400 watts. Considering that my receiver is not really putting out 100 watts, I figure the best I can currently do is about 98-100db. Not exactly reference, but close enough for me.
