I bought a bunch of PF5102s to use as subs for J201. Have not used any yet. Looks like the J112 will be higher gain in most cases. Feel free to mix them up in the M800 if you like. I think Q1 and Q3 will be the most likely to exhibit an audible difference between J112 and PF5102. Start with all of the bias trimmers (except TRIM4) set for 5V on the associated transistor drain and then tweak for best sound from there. Even swapping between two FETs with the same part number will probably require readjusting the bias trim.