I think they used a different manufacturer for the meters, I THINK. They had to do away with the 700 watts nomenclature cause at that time the consumer electronic watchdog ruled the max power couldn't be in the name.
Mostly decoration differences. There are really no major differences since the earliest units in circuit design, variations of the same theme. The 700 and 400 are essentially the same amp except the number of output devices, the size of the bulk cap and the VA rating of the transformer.
Now if you're talking driver board, the B had started with the PL14 board, and also had a PL020 board toward the end of it's run. I don't believe a B had a PL36 board, I believe only series II' had a PL36 board.
Yep. Like Joe and Lee said; cosmetics. This has been a factor to me as of late myself because I have wrestled with having two different versions in my set up. So I have noticed a few details. The Lab standard's were the later incarnation. Some of those differences include things like three bayonet base lamps in the logo area of the lab standard versions instead of the two in the R.M.S. version. Also on the R.M.S. version, shorter heat sink fins, and heavy metal shiny knobs, instead of the plastic pop on D shaft style knobs on the lab standard. Meter attenuation switch reversed, and channel B pot embossing on the face plate faced inward on the lab standard, instead of toward the handle side like on the R.MS. version. And if you care enough about details.......the info sticker on the heat sink about the speaker binding post hook up procedure, was stenciled in white on black with the R.M.S. version. Whereas it was later changed to the typical gold on black we see most Phase amps having. The meters? Well......apparently phase used all three kinds in both versions. Either one is an excellent choice. I mean....it's a 700B. Period. And that's what really matters.