just because a speaker is rated up to 100 watts doesn't mean it will blow up if you feed it 101 or more. That is just a broad rating of its handling capabilities, but it will not melt if there is a short dynamic peak that goes above 100 watts. You really don't have to "match" an amp to the speaker like that, and in fact an amp rating for MORE power will do better because the real danger to your speakers is an underpowered amp clipping when you try to drive it too had.
also, just because an amp is rated at "120 watts" doesn't mean it will be constantly putting out 120 watts. You only draw as much power as you need to achieve the volume you are using... which in most cases is actually much less than you think.
For example, those B&W 685's are rated at 88dB sensitivity; that means they will put out 88dB with ONE WATT of power. It takes 10x as much power to gain 10dB of SPL, so they will be putting out 98dB with only 10 watts of power. Even subtracted some SPL for listening distance, if you actually fed them 100 watts they would be ear-splitting loud; you almost certainly will never need that much power unless you were trying to drive them to huge volumes in large room.
That's why for a 2-ch setup in a medium-sized room a 40-50 watt integrated amp is often plenty of power. Remember also that a Rotel will be rated much more conservatively than a lot of other brands... those "40 watts" on the Rotel are probably a lot more REAL power than a little sh!tbox $299 Sony or Yamaha home theater receiver which advertises "110 watts x 5 channels!".
