I find this all strange. I think we have a pathological science thing, like the tube audiophiles have, going on here.
At HF, all of the noise comes from the antenna and not from the radio circuits. The only exception is when using really narrow filters, some radios may have a little out of passband white noise.
Most complaints or compliments about "noise" are really about AGC charateristics. TenTec for example has a fairly high AGC threshold. It acts almost like a volume expander at low levels, meaning the gain does does not pump up between signal lapses. That gives the illusion of a "quiet receiver".
Other than bandwidth of filters, there is very little that really makes one receiver "quieter" than another at HF except how you set the gain (attenuators) and how the AGC is configured. The single largest problem I see here with guest ops is they fail to use the attenuator pad on receivers when it is needed, and when they use a lower gain receiver they think "ooohh, this one is quiet". Reduce the gain in the hotter receiver and it is just as quiet.
Virtually every receiver at HF is external noise limited. I have not seen one yet that has any better S/N ratio than another if the same bandwidth filters are used, and if the operator knows how to use things like the gain controls, AGC settings, and attenuators. The sole exception is broadband white noise on some receivers that comes in after very narrow filters, but the brain or an audio filter can take care of that.
I don't know what receivers you folks all have, but every receiver here is limited by noise picked up from antennas and so they ALL have equal noise floors on HF. My 1941 HQ120 has the same noise floor as a K3 when they are hooked to the same antennas. I'd bet you are listening to AGC differences.
73 Tom