The 6SN7 and the 12AX7 are dissimilar. Theoretically, they could be interchanged (assuming you’d make the necessary connector changes), yet, they have different uses. Never exchange them If you prefer not to sacrifice sound quality. 6SN7 tubes are low output impedance tubes with low gain and high current. 12AX7 tubes are the other way around- high output impedance tubes with high gain and low current. These properties make the 6SN7 family ideal for line-level uses. They work great in preamps since they preserve the signal with minimal distortion. Alternatively, 12AX7 tubes are best used for high gain. They are suitable for vinyl and other phono-stage use cases.