QUOTE(physdude @ Nov 12 2017, 08:17 PM)
I am using a Yamaha RX-A1060 with 3 KEF LS50s and a KEF HTB2 subwoofer for what is currently a 3.1 setup. I have just finished setting up everything after moving here so it is time to get the rear speakers now. However, I am worried if the A1060 can effectively power all the speakers if I get another LS50 pair for that purpose. The system works pretty well right now and it doesn't sound underpowered at all though every review indicates that the LS50s are very power hungry. Or should I go with a Q100 pair which should be timbre matched?
The better timbre match is actually the Q150/Q350 instead. The Q100/Q300 sounds nothing like the LS50 IMO.
Anyways to answer your question. Yes the LS50s are inefficient speakers, a 1060 at 5 channels driven puts out around 90-100 watts at 8ohms 20hz-20khz full bandwidth (rough calculation from me based on the stupid "JEITA" specification, whatever the hell JEITA means) at under 0.1% THD.
90 watts for most people, is enough. But wattage is related to how far you sit from your speakers, how loud you play and how big your room generally is (although room size is less important in comparison to the sub). If you are going to be doing things like "multi-stereo" for 5 channels at reference volume (good bye your ears) regularly where you sit like 20 ft away from your speakers, then no - 90 watts isn't going do your speakers any justice. You may even hear distortions and worse, clipping.
But there's more to just wattage for "power" as a measurement. Transients and clean delivery is also a factor in determining sound quality, especially when it comes to dynamics and the low end. The Aventage receivers has a fairly decent topology and they do well enough for their intended purposes. But Yamaha do have a hard limiter on their amps (which is why you'll rarely hear or see a Yamaha amp be hot enough to fry an egg) which in some cases, can indeed hurt dynamics.
Amps aren't super expensive now a days though - you have good quality amps at a considerably decent price like the Emotiva BasX line. So my suggestion is, get your rear speakers and hook it all up to your receiver. Hear how it sounds, hear if it's enough to hit your intended SPL, then decide if you need an external amp. Remember, you are also crossovering the sub-bass to your sub which takes the majority of power requirements on the LS50s.
TLDR; The worst decision you can make in this hobby is to buy everything at once and then realize you don't need the extras. Do it one at a time.