Pioneer Vs Yamaha
The 1% THD is not going to be noticeable at all. I have the Pioneer 815 which is identical in the specs with regards to THD and in normal to very high listening I had no issues whatsoever. I have the 815 in my bedroom and my Onkyo in the living room driving my main system.
The 815 and it's replacement, the 816 are wonderful capable receivers. Audioholic did a review on the Pioneer 815 and they were very impress with the unit considering the price vs performance ratio. The difference between the 815 and 816 is that the 816 adds a few more watts per channel (110 vs 100 in the 815) and xm decoding. Everything else is identical. You can not go wrong.
With regards to Jiannisk, I would go with the VSX 915. I have not seen anything with regards to a Pioneer 916 yet. The 915 has better connectivity options then the 516. Again it depends on your needs but the 915 has components inputs, pre out for all channels and will will give you more performance then the entry level 516.
HDMI will become more important in my opinion in the future. The new specs for HDMI 1.3 have not been work out yet so I would wait until then. Also, if your TV does not have an HDMI input it has no value as of now.
I have not used the Yamaha 659, but it has receive many good reviews as well. Again, I tend to lean towards Pioneer because most of my viewing is in movies and they use a double precision Motorola DSP engine that is used in most of the movie theatres worldwide to decode all digital formats.
With that said, both the Pioneer and Yamaha are very good brands. I think- in my humble opinion that you might get a better bang for the buck with the Pioneer brand.
Let me know what route you go and have fun!!!