I use the 402 with a pair of B&W CDM 9 NT (a couple models below the 803 but with the same recommneded power) that has a sensitivity rating of 89 db/w. There is no reason why you would be using all 400 watts of the 402 unless you are seated 35 feet away from your speakers :). I find the 8 ohm tap sounds best for my system. You can't go wrong with either amp as both have plenty of power, but you needn't be concerned about damaging the 803s with the 402. There is a power meter on the Mac (as you are aware) which I'm sure is only somewhat accurate. However, for the loudest SPLs I can tolerate, the amp has never been pushed above 100 watts.
MC402 vs. MC252 B&W 803S
Any comments as to how the Mcintosh 252 does with the 803S vs how the 402 does with those speakers? Will the 803S get damaged by running them loud with the 402 so long as the power guard does not come on? In other words, can the 803S take up to 400 watts ? I know the recommended amp is 50 to 250 Watts at 8 ohms but those speakers dip to 3 ohms at long freqs. Also heard that B&W recommends an amp that doubles output power for speaker impedance of 4 ohms. What ohm tap are you using on these speakers the 8 ohm or the 4 ohm. Any reason to go to the 402 or is the 252 plenty.