Fender Tweed Deluxe 5E3 - squeals

Why does this Tweed Deluxe squeal at higher volumes?
It is a 1954 Deluxe converted from 5D3 to 5E3 long ago by an amp tech. I am not a tech.
The signal goes from the speaker out to a box with a 8 ohm resistor and a pot. The line signal goes to a 1958 Tremolux.
The tubes are just checked. V1 is 12AY7. The voltages are a little bit lower than what I find on this site: https://robrobinette.com/How_The_5E3_Deluxe_Works.htm
It is powered with 110V.
Any ideas are welcome.

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