When I adjust the phono gain to either 52 or 61db. gain and then set system volume to a measured 79-80 db. playback level, the background circuit hiss sounds pretty much the same to me. Don't know what to think about that, confounds my expectations as well.That's not surprising. Assuming no special problems such as ground loops are present, overall noise performance will usually be pretty much determined at the front end of what comes first in the chain, because signal levels are lowest at that point, and any noise that is generated or introduced there is amplified by everything that follows. That is particularly true if the gain of what comes first is high compared to the gain of what follows, as it is in this case even at 52db.
So I would not expect noise levels to be affected in any major way by how the overall system gain is allocated among the various amplification stages in the system.
Regards,
-- Al