The NAD amp you mentioned would be a very good choice. I would certainly recommend it...but:
It is $1300, though, and for $1200 you could get a much better-sounding receiver with plenty of power to drive your speakers; the Cambridge Audio 551R. That is the recommendation I would make to a friend. Cambridge is the Mercedes-Benz of receivers, and their power ratings are far more realistic and conservative than others. IMO the Cambridge 551R would have way more power than you need, but more importantly provide MUCH better sound quality than anything Yamaha has ever made! With the Cambridge receiver, your speakers will sound so much better that they won't be able to wipe the smile off of your face for a week.
It will make all 5 channels sound better. Adding an amplifier will give more power, but not improve the sound of the Yamaha. It will still be screwing up your overall sound quality.
You would NOT need a separate amplifier to drive your speakers with that receiver. It has amplifers and power supplies that have been engineered for good sound quality and maximum speaker drive capability.
Your speakers are actually quite sensitive, at 92 db per watt, and do not require 300 watts at 4 ohms. Even 50 watts at 4 ohms would be plenty of rated amplifier power for those speakers. 10 watts of actual power applied to those speakers will produce a sound level of 102 dbA, which is louder than a live symphony orchestra at close range. I play my system pretty loud, but seldom THAT loud.
Any actual power level over 100 watts would really not be usable with those speakers.
The ACTUAL power output capability of your current receiver is probably about 40 watts per channel when driving 5 speakers (and it should not even be connected to your 4 ohm speakers; it will be unstable with that load).
The 2.6 volts that is specified is only the MAXIMUM voltage the preouts will produce without clipping. Normally they will only be putting out 100-500 millivolts or so, depending on the instantaneous nature of the audio signal. The signal fluctuates with the musical content.
They will work fine with any amplifier.
I'm near San Bernardino here, so hi there...lol.
Edited by commsysman - 12/6/13 at 8:11am