The GT 5751-M is a lower-gain 12AX7 tube type. Stevie Ray Vaughn used these to change his front-end distortion to back-end distortion ratio, giving him more of a “touch-sensitive” output tube distortion.
These test as per 5751 spec with the proper transconductance and current output. The slightly lower gain quiets many amps down. The GT 5751-M is a very smooth front-end tube, but it’s also great in the phase inverter position.
According to Groove Tubes, many Fender Hot Rod series players swear by these tubes.