Can't believe I missed this thread but whatever I'm weighing in.
1. A lot of old school guys used a boost pedal (and a lot of new guys do as well, the amps and guitar choices have just changed). The idea is to roll of the low end and crank the shit out of the mids. This why people put a TS9/OD808 (or equivalent there are tons of them) in from of the amp, dial the gain all the way down or near all the way down, crank the volume all the way up, and set the tone right around noon or 1 o clock (or honestly just set the tone to taste not all amps are the same and the tone knob plays off how you eq the amp). Thundering bass isn't what you want in a metal guitar tone. You get that by adding bass later. The bass should blend in a way that makes the guitar sound full even though when you solo it it's all lower and upper midrange.
2. With what I just explained about a tubescreamer, this means you dial back the gain some on your amp. You want it just breaking up, like rock crunch not what you would dial in for a live metal show. When you put the tubescreamer in front of a crunchy amp with the settings as described it will up the gain quite a bit because you are slamming the preamp (which is the goal).
3. The vintage 30 is a pretty midrange forward speaker, that said I have never been a fan of shoving the mic right up against the grill. I read back in the day (10+ years ago) that Andy Sneap used to cut the speaker grill cloth off his cabinets so he could shove the mic into the speaker even more. I actually prefer to put the sm57 around 3-5" or so away from the grill of my vintage 30s. I usually end up right were the dust cap meets the cone but with a marshall I probably would end up a little further out from center I would guess.
Last thoughts:
Check out circle of tone's video on early Maiden tones.
EDIT: Go ahead and scratch what I said about the tubescreamer. Apparently it was a MXR micro amp and distortion + that you should be looking at. But the idea is the same.
As a lifetime Maiden fan, I am familiar with all the info you mentioned above. Note that Adrian did use the Tubescreamer, but only after '85 or so. Before then, you are right, Dave preferred the MXR Distortion+ and Adrian preferred the MXR Micro Amp (mainly to push the amp on solos, according to a
Piece of Mind-era
interview). Dave is still very fond of the MXR Distortion+, but I think these days he uses it mainly for solos and only in the studio. In addition to those tools, an MXR 10-band EQ with a frown curve and a treble booster (Maiden had them custom made by Pete Cornish, but any decent one should work just fine) were also key ingredients in their tone. I have the TS-9, the Distortion+, and also a Boss DS-1 in my pedal collection, among others, but I don't use them often. In fact, I haven't even opened the TS-9 and the DS-1 box and tried them out. I do like the Distortion+ and its unique flavor, but I find that I can get almost identical results by using just my DSL15C's distortion (I've done the A/B to confirm that). What the Distortion+ does have that is interesting and unique, is that it uses germanium diodes, which are also used in treble boosters, so it has a subtle treble-booster flavor, which can be useful if you want to get real close to the classic Maiden tone, but with the minimum number of pedals.
Overall, my goal is not necessarily to recreate the exact tone, but to get the main characteristics right by going straight into the amp: A tight bottom end, thick and focused midrange and upper midrange, and no fizz. The best example of that tone is on Maiden's cover of "Cross Eyed Mary" (Check it out on YouTube. The way the guitars sit and are complemented by the bass, as you mentioned, is amazing, as is everything on the
Piece of Mind album). Side note: The DS-1 does an impressively nice job of tightening up the low end.
I am somewhat familiar with Mr. Sneap's method. He generally likes to place an SM57 on-axis, just off-center, less than an inch, to "get rid of the 10k fizz," about an inch off the grille. He high-passes and low-passes guitars at 60Hz and 12kHz, respectively. It seems to work for him. His recent work with Saxon and Priest is very impressive and very modern sounding. I can, however, definitely tell that the guitars were close-miked. They sound warm/bassy and a touch scooped. I would prefer a little more definition on the rhythm guitars, but it's probably a trade off for the mentioned advantages of close-miking. To me, his results are the kind of stuff that is fairly straightforward and obtainable. The classic-metal stuff, on the other hand, is appreciably more difficult to cop.
Anyway, it seems to me that I get the best results by backing off the mic around 4–5 inches from the grille cloth, with the mic placed roughly halfway between the center and edge of the speaker. It works really well and sounds very good, without that nasty, woofy boominess that I get from placing the SM57 within an inch from the grille, and the guitars sit so much more naturally in the mix with the mids more filled out, since backing off the mic also reduces the fizz that you get from extremely close mic placement. I have no idea how guys who mic very close don't run into issues with definition resulting from the proximity effect. My only guess is that they must high-pass everything very aggressively either on the console or "in the box" at the mixing stage. Else, I really don't see how anyone could get usable raw tracks without some major EQ-ing in the post.