I don't know how carefully the compression check was done but I cannot envision a good compression check with a loose valve seat.
You didn't mention whether you have a noticable miss at any time which also kind of eliminates a loose valve seat.
If the ignition wires are originals, I find it questionable that they are still good.
The normal carbon-treated twine gets brittle and starts to break up eventually weakening the spark to the plugs.
Have you pulled any of the plugs to check the condition of the tips?The insulator should have a light brown deposit on each of them if combustion is OK and the valve stems & seals, along with the rings are still tight.
I'd also check the spark intensity visually by pulling each of the wires and allowing it to spark to ground unless you want to pick up a plug tester which creates an extra stretch for the spark to jump.
I don't really like that 'loose valve seat' diagnosis but can't be there to drive or inspect the car.