Took out an old bass head from my dad's basement (an Earth B2000) all tube sweet tone through an SWR golliath Jr 2x10. I will need to replace the tubes. The originals are 6L6 GC. When I looked to pick up a set I saw all kinds of designations like GT/GE/C/S/R and WGC.. What the difference between these ??? (please help !) Also they come in sets of 2 and 4 (I need 4) and are designated in degrees of Hardnes from 1-10 what's that ? Thanks in advance for your help.