With the Sriver (and older BTY) designations, Regular is Sriver, (some places real old Sriver L) that has the least soft sponge, then EL is mid soft, then FX is mushy soft.
In the old days of speed glue, the FX could really soak up the glue and light it up, but now unglued in stock form, I wouldn't use FX or EL if you paid me to. Regular Sriver isn't all that firm to begin with.
It doesn't matter WHAT rubber you slap on Ur setup for blocking if you do not see where the ball is going and be there on balance to take it off the bounce, or away from table with touch (using less wrist grip pressure). XP 2008 is middle of the road in everything and I know what I am getting. I can block with just about any inverted rubber, it is just a subtle change in blade angle and grip pressure and force (less or more) with the different inverted rubbers. Some rubbers are more sensitive to spin with passive stuff (Think TENERGY) some are relative non-reactive to incoming spin to a degree (XP 2008) (ANY old used rubber that is losing grip)