Im wasting my time here.......................
Again, the "EVH in a box" thing really is like asking scientists when the flying car will be made. I can make all sorts of suggestions for OD's, hell I'll make some now based on what people have used over the years. The MXR Distortion+ was popular for copping Ed's tone back in the day. Apparently everyone used them to get semi-close while using similar gear. There is some talk about it on the Metro forum, Ralle made a clone and tweaked it to sound closer. I once read a suggestion for a Jordan Bosstone without the clipping diodes. Sounded okay. Obviously the more plexi-voiced overdrive pedals previously mentioned here will do a better job.
And yes, there WAS a distortion/OD in front of Ed's amp. The EQ pedals he used had a whole bunch of gain on tap. Though I've never personally used either the MXR or the Boss GE-10, I built a BYOC EQ for kicks a while ago, and it had a very pleasing (but noisy) overdrive to it. Didn't sound like anything else, just sounded like my setup, but with more push to it. This was a totally different circuit, but what I'm getting at is that's a big part of the sound. Now look at this picture:http://wwwc.dcns.ne.jp/~epi/EffectersPhotos.jpg
The pedalboard, pictured in the top right, has the MXR EQ on it that particular day. Check out the sliders. At least one slider is up all the way, and none are on the 'cut' position. The pedal claims +18dB of gain, which is a LOT. The GE-10 has something like +10dB. Either of these pedals hitting the front of Ed's amp was big-time distortion. Since he also tended to crank his plexi up all the way, and supposedly used rock standard Greenback speakers, those speakers would be creating tons of speaker distortion. If you listen to AC/DC, you know a bit component of the sound is the speaker distortion. Ask anyone who's tried to get the Angus sound at bedroom levels
One thing I can mention though, since a few of the Isolated guitar tracks have come out some have said they don't really hear a PAF. They hear a Super70 (standard in the Ibanez Destroyer) and possibly a Mighty Mite Distortion pickup. Both of these are higher output than a PAF. Ed has said a few times he was disappointed with his tone on the first album, and there is a possibility that he went to a lower output humbucker for the second album. That makes sense, since that signature VH1 drive is gone from the second album.
And don't discount superlead's just because they didn't do it for you, "modded" or not. No two Marshalls are alike. Parts substitutions, in value and type were just how things were done back then. There is NO complete circuit "evolution" because there are always exceptions to every theory. Factor in that components can sometimes drift in value, and you've got a wide range of possibilities. And just saying a plexi is modded is very unspecific. Like most vintage circuits they are very simplistic, so the mods could really be anything. Another thing to consider is that preamp tubes will shape the tone quite a bit, and there's basically no information about what Ed used.
Of course, all this will only work for you if you're looking for the live tone. The studio was a big factor in the recorded sound, as has been with every coveted sound in the history of Rock. Those who use slaving say they love it for the studio tone. I'd give that a shot before trying the endless stream of pedals on the market. Once again, I don't personally believe he slaved, live or in the studio, until later albums/tours, but it sounds good.
As unfortunate as it sounds, unless you're 1978 Ed himself, you'll never sound like him. By all means chase the sound, but don't expect miracles.