Electro-Harmonix 12AX7 Preamp Vacuum Tube
The Russian Made Electro-Harmonix 12AX7 tubes are popular with Marshall owners (and also similar amps) for their gain and bite. They are the tubes that are already found in a lot of amp brands on the market for their sound and reliability.
More pronounced mids to high mids with a tight bottom end and higher gain. These tubes work well in amps that have a large bottom end.