If we think a typical marshall plexy-jcm800 phase inverter I think it with the typycal resistors 470 ohm from cathodes tied together,and the 10 kohm and then 4.7 kohm to ground.
With the introduction of the JCM900 the typical phase inverter has changed the values from 470ohm to 1 kohm and 10 kohm to 15 kohm.
Do you know what electronic things change varying these resistors ?bias point of the PI ,gain,ecc ecc ???
Speaking of sound and feel the change from 470 ohm to 1 kohm sounds to me more "cold",more linear and a bit more "metal sounding" .The raising of the 10kohm resistor to 15 kohm or 22 kohm seem to give more bass frequencies
Also the 47pf between anodes in we raise it to 100pf or more,make the sound more hollow,less picky (actually reducing high mids and higs frequencies..)
But ,the real question is....
Why marshall did that in the JCM900 series and 6100 amps? For the different type of output transformer used (C3070 of the jcm900 is a bit hifi interleaved,C2668 is NOT interleaved,only sandwiched...) or for different sound era?? For the different power tubes ? (In the 90' marshall uses a lot 5881 because EL34's sucks in terms of reliability in that era....but the JCM900 began with EL34....!!!! ) ???
