Power Filtering vs Power Regeneration?

I didn't realize that you were down in Sydney. Don't the records want to spin counter clockwise there? 😊

Have you noticed if the degradation occurs during wet or dry spells in the weather?

I asked that wondering if it is possibly related to grounding. Ground rods deteriorate in some regions and also arid ground (dirt) can be less conductive vs. moist ground (dirt). And another thing with tube gear, humidity can sometimes affect the sound.

RFI is what I was thinking about when asking about the grounding. Poor grounding could allow RFI to muddy things up. Also, I was recalling that in the 80s a buddy of mine had some old (50s vintage) tube mono amps. I remembering him remarking that humidity affected the sound. Now that I am thinking about it, he also had Quad Electrostats. So that might have been why humidity mattered. On a not so related note: I notice that my system sounds better when the air temp in the living room is around 68F. In the summer when temps are in the mid 70s in the living room the sound is not quite as perfect or astonishing.
One last thing- I have no idea which way the records spin. I listen to records in the dark.
Ok, one more last thing: wide variations in line voltage is a big issue, Have you complained to the power company about it? I'm sure that the wide variation in voltage will impact life/durability of electronics, motors and lights.

That's a 4% total variation in voltage. I'd say its a big deal. I would expect less than 1%. Now, that's considering that the load in your house is constant. If something in your house is turning on/off when you see those voltage swings then it might be a problem with your house wiring and/or you are too far away from the transformer. I see 125VAC at my house. It stays within a few tenths until a load comes on. If both heat pumps are running and other things are on then I see it drop to 122.5VAC. When I power up my monster amp it gets as low as 121.5VAC but never over 123VAC while the amp is on. 120.5VAC is the lowest I think I have ever seen and that was summertime with the stereo on and A/C running. I have a dedicated circuit for it. It pulls about 5 amps idle current and the preamp/CD player, tt pull about another 1 amp. The power transformer sits at the street at the corner of my front yard.