One reason for the device was telephone switching equipment, the Fetron uses less power and last much longer than vacuum tubes. Other technology replaced those switching circuits so the Fetron did not last so long.
For a guitar amp, you would have a reliable, consistent, quiet preamp device. It operates at a lower voltage than a 12AX7 but still high compared to a regular transistor. There is a selection switch on the bottom of the chassis to select the correct voltage.
Both FET (field effect transistor) and vacuum tubes are voltage controlled devices. In one sense, they would perform similar in the linear region and sort simliar in non-linear (distortion) region - even order harmonics. Bipolar transistors operate as current controlled devices and deliver odd order harmonics when driven non-linear. There are other differences, the device and the circuit surrounding contribute - compression is one.
If the front end device is meant to be clean and not pushed with a pedal, it may make little difference in the tone. People do hear differences in tone with different tubes so you will have to listen and decide for yourself. Note that the Boogie Graphic EQ is a bipolar transistor design. The juicy tones are generated mostly in the preamp tubes past V1. The EQ has lots of signal headroom so it does not add/subtract tone except for the frequency sculpting. Then the output section adds some flavor, more when pushed.
