The power transistors, even older ones, are small and light in weight. The heavy and expensive parts of receivers or amps are the power supply transformer, the heat sinks, and the metal chassis.
Correct. All the new development went into improving the processing systems for digital surround sound. The only development of amplification (stereo or multi-channel) was to manufacture low-distortion high-power systems at a lower cost. In that sense, the cost of powerful amplification has gone down over that last 20 years. At the same time, the cost of high quality speaker systems has gone up.
Actually the opposite may be true, but I don't expect to hear that from owners of older gear, even if they don't want to sell theirs.
Those differences in power, 145 watts vs. 120 watts, and total harmonic distortion 0.01% vs. 0.005% are too small to be heard.
Doubling the power is barely audible, an increase of about 3 dB. To make a genuinely audible difference in power, one has to increase the power by at least 3-fold to 10-fold.
Most loudspeakers, even very good ones have distortion in range of 1-10%. As long as an amp has THD below 0.1% it is said to be low distortion and clean sounding. Yamaha in general has a long-standing reputation for excellent amps and receivers.
All this talk about power and distortion depends on the speaker being used and the amplification levels required by the room. You've never mentioned speakers or room size. Unless your speakers are particularly difficult to drive, or you live in a large auditorium, any of the Yamaha amps you've mentioned should be very good.
