WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
A 6SN7 is one of the most linear tubes out there if used properly. Bias it to between 7.5mA and 12mA for lowest distortion and best operation over time.