These people are being nice. My specialty is brutal honesty: The AVR is the absolute bottom of the audiophile food chain. They are crap. Pure crap. Every single one of them. Regardless of price.
Now with that out of the way let me explain exactly why, so you will understand that this must and always will be the case.
The most fundamental facts of audio are that quality matters more than quantity. Everyone, but everyone will prefer the sound of the high quality 20 watt amp to the crappy 200 watt amp. One high quality rectifier or cap will improve the sound quality so much you won't believe. (I've actually done this stuff, so have seen and heard it first hand. The difference is big enough to be obvious to anyone and everyone. Its not at all subtle.).
Another fundamental fact of audio is every connection, every bit of wire, makes a difference. This is sort of a restatement of the first fact. Repetition helps. You can improve the sound of just about anything very obviously simply by removing one or more connections making for a more direct signal path.
These facts explain why it is that so many people rave about the sound from simple tube amps and other components with really elegant simple designs. Also explains why it so so incredibly expensive to get a high powered amp with a lot of complex circuits to sound good. It can be done. But you might prefer the house you could buy for it instead.
One more fact. Noise comes into circuits all over the place. Electricity is electro-magnetic. That is to say you cannot have an electric current without also having a magnetic field. Electric currents produce magnetic fields. That's just what they do. Whenever a magnetic field crosses a wire it induces an electric current in the wire. This is how transformers work. This is how electricity is generated in the first place- some power (water, wind, whatever) turns a big magnet, generating a current in the windings of wire around the magnet.
This is a big deal. Because inside your AVR is a mess of magnetic field (read: noise) generating electronics and wires, all stuffed in there tight as can be, a veritable rats nest of wires.
If you want to get some idea just how hilariously stupid and bad an idea the AVR is have a look inside any really good phono stage. The phono stage takes the weakest signal in all of audio, applies EQ, and amplifies it more than anything else in all of audio. If there's any noise at all its gonna be horrible. Phono stages show how it should be done: simple, direct, and organized to keep all the noisiest parts away from the signal as much as possible.
Exact opposite of what you find under the cover of an AVR.
Sorry, but they are just a joke to me. Its cruel cold humor at your expense to try and trick you into thinking there's anything "direct" about plugging your interconnect into an extra set of connections, only to have the signal go right back out again through another set of connections, and then through yet another interconnect. Meanwhile along the way exposing the signal to the greatest concentration of noise you have in your whole house! They should be ashamed.
Why can’t the avr producers do a better job when designing their avr’s or maybe not a big deal in that avr SQ for stereo is comparable to a stereo preamp.
Well now you can see, they can't. Even if they wanted to. The result would be a box the size of your whole audio rack, and cost ten times as much- and even then not be as good as the tube integrated you could buy for much less. They get away with this because they are usually used to watch video, and when people are looking they generally aren't listening. Close your eyes and see. Also they have a million, sorry make that a billion, reasons, called dollars, to stuff advertising in your face 24/7 to convince you the worst component in all of audio will do you just fine. Absolute junk has been and always will be highly profitable.