The first fiddle, the "Student Fiddle", was...ugh. Those screeches, squeeks, and pops. My first thought was, "Can I use this for dubstep?"
The "Standard Fiddle" was a dandy fix, and is a good starter fiddle now.
When the Fab Four Fiddles came out, separately, I wasn't quite sure what to make of them. But after getting all four, and playing around, I love them!
Each of the four fiddles is actually one fiddle, played four different ways:
The Travellers Trusty Fiddle is Plucked. The sound created by picking a string off the neck with your forefinger and thumb, and dropping it back so that it slaps the neck. On other instruments this sound would also be created by finger-picking. This is great for imitating a Banjo, especially when matched with a Basic Lute. I've also used it for glockenspiel, and other instruments too high for MMH.
The Lonely Mountain Fiddle creates long, sustained, vibrato notes, that increase in volume with the length of the note (to a point). This is the long, slow draw of the bow across the strings, while vibrating the strings with your fingertips on the neck. Medium notes are quiet, and quick notes are imperceptible. Gotta watch the volume on this one.
The Bardic Fiddle creates sustained notes, and is the backbone of the Fab Four Fiddles. This fiddle is the equivalent of the Horn, an octave higher. Anything you have played on Horn that had to be bumped up a notch in Maestro, is normal for the Bardic.
The Sprightly Fiddle gets it's sound from hitting the strings abruptly with the lower end of the bow (more control) to create a short, brusk note or chord. This fiddle is LOUD. I usually will dial back the volume in Maestro all the way to just above zero, usually.
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So...you want to play that one song that has that really dark, chunky, distorted heavy metal guitar sound...here's the simple version (it can be complex, depending on the song)
Say it's an easier song, with four parts to the midi: overdrive guitar, bass, drums, atmosphere.
Drums are drums. But crank the volume. If it's too loud, you're too old. Much of hard rock/metal is the kick of the bass drum to match the snap of the snare, you miss that at low volume drums.
Atmosphere, or whatever they're using for vocals is usually easy to pick an instrument for, vocals (being human characteristics) are almost always within 2 octaves at the most. I like to match the instrument for vocals with the style/sound of the vocalist. Johnny Cash and Peter Steel are Horns. Dio and Blondie are Flutes. Dave Mustaine and Axl Rose are Bagpipes. Most everyone else is Clarinet. But that's just me, pick what you think sounds best!
Bass. You can use the Theorbo. That's what it's there for. 
But you can also pair it with MMH to give it more "thump". You can even add to that by matching both with Basic Lute, dropped. Lots of Bass pop and thump for one track! But you can also just use Theorbo. Theo's are really well done and can handle things on their own for sure.
That guitar. It chunks, it screams, it roars. So it needs three instruments for the masquerade to work. (Most of my "metal" songs that I do match 2 pairs of Bardic Fiddle and Sprightly Fiddle, but some require more than that).
The chunk (usually caused by hitting the strings with a pick while slightly muffling the strings with the palm of your picking hand, or just the straight up chunk from the heavily overdriven amp) is the Sprightly Fiddle. Short chunky notes. Remember, it's a LOUD fiddle, so drop down that volume almost all the way, you can raise it up a tad after adding the two other instruments.
The roar is the Horn. It's what you used for distorted electric guitar previously, and it's awesome still. Those low E5 G5 E5 A5 E5 D5 E5 chords that are the first chords in Dirty Deeds, yeah, that needs Horn.
The scream is the Bardic Fiddle. While the Horn may be the 454 big block, the Bardic is the 350. The Bardic will give you the short, high notes needed for the solo, along with it's normal sustain during the rhythm. And yeah, for many of my metal songs, I use just the Bardic alone, it's still a big block. You want a 2.2L with turbo, that's a Clar.
So, I use all three instruments matched up to make one guitar sound. Not saying that this is the only way to play at all. I love how everyone can sound different. Try everything and see what you like best!
