It could also be argued that a full rrp avr costing £600 now is probably not that far off something that's gone eol for a few years costing 100's more
(some prefer the Yamaha "sound", others prefer a different presentation - each to their own imo)
Ie the same componants in the Yamaha would cost less in a retail product today (if they were available)
Features improve at certain price points, that's for sure. But some important stuff such as large transformers and big capacitors and fast high-current-flow transistors have already reached the point where manufacturing costs are the lowest they're going to get unless production volumes shift significantly. Also, the cost savings from moving manufacturing to low-wage economies such as China have all been realised. The bottom line is that quality costs.
What we've seen in AV Receiver/AV Amp production is pendulum cycles where the attention shifts from advances in technology to focus on price/performance and then back to technology and so on. It's whatever the manufacturers feel that will give them a competitive edge at that point in time.
Currently we're in the tail-end of a technology phase. There has been a lot of attention on 4K/HDR/ATMOS as these are trending with consumers. We're seeing a lot of effort going in to trickling these features down the price ranges. As a result, economies are being made in other areas. Take a look at the back panels of most sub £600 AV receivers and you'll notice that each year there are fewer analogue, optical and coaxial inputs. In part, that's driven by changes in source technology; we just don't use analogue and coax/optical as much now. But that shift happened a few years ago, accelerated by HDMI ARC and the shift to streaming; so why the delay? The answer is that as 4K & ATMOS trickles down to the lower reaches of the AV receiver product ranges, there's less cash in the pot to pay for the tech required and still keep the legacy features.
Along with the denuding of the back panels you'll also notice that there's a shift in some quarters toward using Class D switching amps rather than transformer-based amplification. Manufacturer's will spin this as the drive for greater power output or that digital amplification has come of age, or higher efficiency depending on whether the story is for end consumers or bureaucrats with a green agenda. Behind all that PR hot air the simple fact is that Class D is cheaper to build. When the hard choices have to be made then more noise and less fidelity is an acceptable trade-off for 90% amplifier efficiency at 1/3rd of the cost of a traditional transformer-based power amp stage when trying to squeeze a quart from a pint pot budget.
Pioneer have gone heavily in to Class D. Onkyo has some Class D AV product. Denon uses it in the Heos multiroom amps. All the brands that produce mini-systems and all-in-one kits use Class D. The other way to fudge higher apparent wattages is to start quoting on 6 Ohms rather than 8, and by measuring at higher THDs and at 1kHz rather that full audio spectrum.
Bringing this back to your sound quality comment; advances in source quality such as the shift away from lossy compressing in DD and DTS to lossless in the HD audio formats, combined with smarter auto calibration including room EQ to help reduce the incidents of truly awful set-up have (IMO) been off-set by the damage done through cost-cutting on hidden tech to pay for headline features. I'll give you two direct examples. The first is from when I swapped out a customer's old high-end Denon for a new Pioneer. Price-wise they were comparable if you ignore inflation; ball park £1200-£1400 a piece. On paper, the new Pioneer was way more powerful, and so far ahead in terms of features there was just no comparison. When we fired up the new amp though it sounded noticeably quieter and had a thinner- and less involving sound. That's because the Denon flowed a lot more current.
The second is at the other end of the price spectrum. I have a customer for whom I swapped out a late 90's Yamaha AV receiver for something newer with HDMIs. Most folk wouldn't have given the old Yamaha a second glance. However, I knew it was equipped with a very good power supply and high quality capacitors. The low noise floor and dynamic ability meant it could embarrass some well respected 2 channel Hi-Fi amps in the £500 price range. It would blow today's AV receivers in to the weeds on music and well set-up DD/DTS. I sold it on to a customer who was looking for exactly this sort of high quality performance in an amp that supported digital as well as analogue connections.
Before you get the impression I have a downer on modern gear, I don't. What I am is realistic about manufacturer claims and equipment reviews. The fundamentals of amplification really hasn't changed that much. Amps modulate the mains and supply current. On the whole, this takes a heap of cash to do that well.
