I've got three Genalex Gold Lion gold pin ECC83S preamp tubes installed in my Marshall MA100C amp and they've been really good so far, I bought them online from Evatco, they are a little expensive but the way I see it, it's worth paying a bit of extra money to get decent tubes for guitar amps since cheaper tubes are only going to give you trouble later on down the line....definitely don't skimp on tube quality.
https://www.evatco.com.au/
https://www.evatco.com.au/12ax7gg