I would still like to address this issue, but noise theory is not oneof my strengths. I do think that what I observed is related to noise analysis of OP Amps. See
https://www.electro-tech-online.com/custompdfs/2010/12/sboa066a-1.pdf
The amp I built was essentially a big OP amp. It is a varient of this **broken link removed** designbuiltin the early 80's. One of the noise notes that I came across says to reduce the overall noise of an OP amp, you need to reduce the feedback resistor. Reducing the voltage gain would have a profound effect when operating at low signal levels. As a tweak, this would be one improvement that I would add. The larger the resistor, the larger the contribution of current noise. I do belive, but cannot prove, that the AMP when operating into an 8 ohm load the voltage noiseis the dominant term. When the large resistor is added to the output, the current noise becomes the dominant term.
I did do some laboratory tests that measured currents using the voltage drop across various resistors using a sample/hold kind of A/D converter and saw the noise. I wish I had the plots. I-V converters do a much better job in reducing the noise. I designed a 4-terminal version of one of these.
Tp some what back up my experience. Back in the 70's my parents bought me an all in one kind of stereo system. It came with speakers, headphonesand a ceramic cartridge.
I won speakers and put them on the system which souded OK on the speakers it came with and the hiss drove me nuts. I built a small AMP with a switching power supply that ran off of 12 Volts and the sound was much better. I then abondoned the stereo all together and found a broken AMP at a white elefant table, so I used the pre-amp section of that. I bought a turntable and had to use an external pre-amp all because I had very little money.
The headphones were the same way. Higher quality headphones yielded hiss and actually made the sound worse and it used the series resistor thing for the headphones. The same thing happend in my car. The factory speakers sounded good, but I upgraded them to plate speakers, the hiss from the tuner was horrible.
The noise is real when you use a series resistor in a high quality amp with a freq respose of 0.5 Hz to 40 Khz. If you can hear it depends onthe quality of the output device(headphones).
There is a MAJOR difference between Koss Porta Pros and a Bose Quiet Comfort II. There's a big difference betwen the II's and the QC III's. The Koss Porta Pros sound the best. Probably the best headphones I had the opportunity to listen too was a set of Electrostatic headphones by Stax such as these: **broken link removed** They will set you back about $3,000 USD.
My friend and I did a side by side comparison of my amp and a McIntosh Tube amp diving Klipsh horns and we both agreed that mine had much better bass than the tube amp. The tube amp was louder driving the horns. The quality of the sound was about the same. We also discovered that I like folk music and it sounds better with a dome tweeter whereas, my friend likes classical music. My friend built a few tube amps.
My preamp has a -3db point at 100 kHz. M y eqipment is not typical audio. It's high end. The preamp actually has 3 voltage gain stages and each stage gets a volume control. That's not typical either.