He ran an EQ and the Echo before. That is pretty much clear and documented... but if you run phasers before a dimed Marshall, it would sound like poo. That's why he used the power amp, so he could run effects between the dirty amp and the clean power amp, otherwise why would he run the power amp? Obviously, those old amps did not have effects loops, so the power amp acted as a clean channel to add the effects over the guitar sound.
VHII didn't. It was just the Marshall and Donn Landee added the effects afterwards and most records after that were just combinations of the first two records' techniques.
This is from Ed's words and Ted's interviews in the late 70's, early 80's.