1) Er, no. There are class A amps, class B amps, class C amps, class D amps and all kinds of hybrids and variations and whilst, yes I agree that a sine wave put through one should come out the same as a sine wave put through another, you are ignoring the complexities and nuances of intricate musical signals, the frequency responses and attenuation characteristics of different electrical components, circuit board layouts, power supplies, etc. If not, we'd all be buying our "hifi" at Argos, and WHF magazine would have disappeared off the shelves long ago, right?
2) Economies of scale? Whilst it's true that a small scale manufacturer like Cyrus or Naim or any other small volume brand you care to mention has to divide their overheads between only a very few units comapred to the big boys, this is manifest in the exponentially higher prices of their products. Brands like Sony, Yamaha, Onkyo, Pioneer, Marantz, etc. sell their units on a global basis, more or less only changing the power supplies, plugs and the language of the knob and button labels to meet local conditions. When you divide out their development costs over the huge numbers of units they produce, development costs are a miniscule part of their price. That's why they're such good value.
If you've got the same fixed target retail price for your black box, whether it be a receiver or a two-channel amp, you've basically got the same fixed amount of money to spend on components. If more of that money goes on HDMI sockets, RCA sockets, speaker sockets, AV DACs, 7 channel amplification instead of 2-channel, licence payments for Dolby surround sound codecs, DTS codecs, etc. etc. etc. it de facto means there's less to spend on the components inside the box that actually process and influence the sound of the 2-channel amplification.