Yes I know there is an only tube thread, but I didn't see the answer to my question.

I am planning to retube my JSX soon and am looking at 4 el34eh tubes. However they are sold in soft, medium, and hard. What is that referring to?
Fender 60th MIA Strat
Epiphone Les Paul Plus

Peavey JSX

Rockman Stereo Echo
Rockman Sustainor 200
2 Rockman 12-Band EQs
Rockman Stereo Chorus
RE-20 Space Echo
fulltone Clyde Standard wah

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