I saw in another thread where you posted about cranking the MV in some of the HBE models. I tried that last night in fact. Im still unsure at times about correct MV settings for various models though there are no rules i know...and I struggle to keep up to the changes that new FW bring. a new FW comes out and users be saying 'yeah so now I change my MV setting to 'xyz' when before it was 'zyx'
When I first started doing this I would generally take the MV to 8 or 9 on a Friedman and reduce the gain knob to compensate, then I'd use a movable mic sim (the DynaCab is of course the best option now, but I used to use Overloud TH-U, Wall of Sound, and Amplitube 4 (whose cab sound I tend to favor over the more reflection heavy Amplitube 5) for this) to find a spot for two misaligned mics on the same cab that smooth the mids and highs to be pleasing, with whatever character I wanted at the moment. More recently I've gone to about 6 on the MV, keeping higher gain on the preamp, and I've even created some tones with lower MVs on bright IR mixes. I can't stress enough that changing your mic position is essential when you mess with MV, because of course your whole frequency range can change. Your sweet spot on the speak then changes too, and it very likely might not be the same as your initial position. If your working with legacy IRs, I'd then audition ones that are captured at different positions, but I'm most in favor in that instance of taking two different mic IRs, misaligning them in the cab block, putting the thicker sounding mic 3 - 15 dB lower in the mix (giving you a very wide range of present but not harsh mids), and continually tweaking alignment (on the front panel for that one parameter, for much greater fine tune control) as well as the level of the second mic, until your mids are just right. Once your mids are right, the rest is easy, just hit the bass and treble until they do whatever you need: if you're in a mix (live or studio), lower the bass until you have plenty of room for your bassist and the kick drum(s), or if you're just practicing on your own, crank the bass until your balls rattle. For the highs I like then to see what they're actually doing. With a lot of presets I've made, I've found the treble and presence controls really to be affecting high mids, so I'm really careful about noting when either control seems to be the source of harshness that I can't control. If I really wanted a treble and presence stack that I can fully take advantage of, movable mics sims with two out of alignment mics are my favorite way to get there really quickly.
I think proper MV values are totally contextual, like all of audio engineering; on top of that, I think there's always room for creative use of wrong settings too, like all of audio engineering. I mean, for a while I played with the 5150 and 5150-II lead channel models with the MV up really really high, and it was awesome! If you take the lows out with the bass and depth to zero, I found really really badass hard rock tones, the kind of amp so many cock rock gunslingers would've killed for in 80s. The thing with 5150s are this inherent mid scoop, but the MV will get rid of that in a second, if you're careful to control the bass. The inherent mid scoop of course makes them ideal for metal, especially with an OD, but it's easy to forget how great they are in a rock context too.
But to me the very best way to get ideas about settings if you're stuck is to find YouTube demos of the real amps. Since you already have the best and most accurate modeler, you can dial in where a video is set then compensate for potentiometer variance as well as your guitar by ear. I use this kind of thing to ground me so I can have a fixed goal then abandon the tone hunt in favor of playing haha. But you may not need an external force like that to discipline you! I would just search for videos where the player really knows good tone and has tone in their fingers too, to give you an idea of what to go for, and you'll see what MV it takes with the real deal. Anyway, this is just one way to go.
