Eds plexi had a master volume added after they recorded van halen 1,I had many conversions with jose arrendondo(Did all of Eds amp work-mods in the 70s and 80s)jose explained to me that during the recording of van halen 1 there were no mods performed on Eds plexi but a overdrive-distortion unit was used,jose explained to me the he(jose)was not sure what type of unit(overdrive-distortion)it was,could have been something homemade by rudy leiran(Eds first tech),if you look at some of the early photos of van halen 78 japen tour photos were you see Eds effects on the floor there are two unit-boxes with the #1 on the box and a #2,one looks like a electro harmonix big muff and the other a old marshall fuzz unit,besides Eds usual effects,phase 90,flanger, ep3-echoplex and the univox echo unit.The box #1 and box #2 were never seen again since then,Ed used the boss ge10 eq and then the mxr 6band eq for the size of the room(venue they were playing)they were playing not for gain,also for guitar changes,the eq unit will not add the type of gain-distortion that Ed achieved on van halen 1,i have also tried those units.