Hi cman,
You may need to ensure that all of your levels are balanced correctly. Also, you cannot run Presence speakers and Surround Back speakers simultaneously on the RX-V1400 - even with a separate amplifier.
As for watts, the output is the same as for the other channels, though both Surround Back and Presence speakers are hardly power hogs. On earlier Yamaha modles the presence speaker were powered by less than the main channels, but since they are now alternating the same amps for Presence/Surround Back they match.