You have B & W Cm series loudspeakers. The Cm9's have an impedance curve that digs down to 3 ohms and I understand why you are thinking of using external amplification.
Everything depends on the type of audio source, your listening distance from the Cm9's, the loudness you wish to listen to your system. If you always listen at a reasonable SPL, it's possible that you might not need external amplification. I suggest that you check this link to find out:
Bear in mind that, as indicated in the manufacturer's published specs, your SR6014 would output 77 watts/ch with all channels driven, and there's no reason to question an insufficient power on the surround channels. For all practical purposes, the possibility of an audio source requiring equal power for all channels is null. Most of the power required is on the 3 front channels. The only exception is if you used the "All Channel Stereo" playback.
As I already mentioned, it all depends on the size of your room, the SPL and your distance from the speakers. But after reviewing the results from the above link, you calculate that external amplification is preferable for additional headroom etc, I would recommend the Monoprice Monolith 3 or Monolith 5 power amp that would amply suit your needs.