I agree with sir Tony to try first before buying unless you plan to have a collection of tubes laying in your drawer....
I tried several 12ax7/ecc83 in my fisher int. amp, both at preamp & PI positions. I didn't hear any distinct difference between these tubes except when I used 5751 tubes at PI (a gift from Fr. Jim aka nizmo). This tube has lower gain than 12ax7 but music has more body than other tubes I tried.
what many do not know, the mu of the tube is not its voltage gain when used in a circuit,
a mu of 100 is seldom translated to voltage gain of 100 in an actual circuit,
the tranconductance of the tube or its cathode current has more influence on voltage gain...
the 12at7 which has a mu of 60 and the 5751 with much of 70 can can have the same voltage gains
as the 12ax7 depending on how it is used in an actual circuit...
mu is like pi of a circle, it is not a physical thing, but is the ratio of the circumference over its diameter..
plate resistance is a physical thing, it is the distance between the plate and the cathode,
transconductance is also a physical thing, it is the quality of the cathode material used
and the heating element or simply the filament...
plate resistance varies with plate voltage as tranconductance varies as the cathode current..
mu is not constant with any tube and also varies with plate voltage....
mu equals tranconductance times plate resistance...
i hope i did not confuse you more...
btw, tube rolling works because the specs of the tubes are merely statistics,
different tube manufacturers make tubes differently using different materials,
and so no one knows exactly how these tubes are with respect to published specs or datasheets...