Subtractor
The Subtractor is about as bog standard as virtual analogue (VA) synths come. It has all the basic controls any synth should have, and will give you all the same sounds that any VA synth will give you. The only aspect that separates the Subtractor from many other VA synths, is the set of phase modulation controls available.
Subtractor is good for sounds such as: simple basses, pads, strings, brass & wind instrument emulations and basic sound effects. One should definately explore using the subtractive and additive phase modulation controls if planning to use the Subtractor.
Malstrom
The Malstrom offers far more than the Subtractor in many ways. It has better lfo options within the mod sections, the filters are superb and the oscillators are based on a more interesting style of synthesis. The oscillators in themselves can create more varied sounds than VA synths like Subtractor.
Malstrom can achieve fatter basses, more interesting pads, weirder effects and better lfo driven sfx than Subtractor. Of course, the interface is slightly more confusing with all the extra options and power.
Thor
Thor is the daddy synth of Reason. It has more synthesis and filter types than either the Subtractor or Malstrom, and it has far more modulation routing options too.
What is Thor good for? Any sound you can think of basically.
Once you get your head around the interface, the sound generation methods and the filter types, the world of music is your oyster!
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Which synth would I recommend for what?
I would recommend using the Subtractor if you are new to synthesis, Malstrom if you are more comfortable with sound design, and Thor if you are well versed in audio production. While there is nothing stopping you from using any or all of the synths, if one doesn't fully understand the synth being used, then it can lead to a more frustrating user experience as you try to make a song.
If I use Reason at all, I personally tend to use Malstrom, simply because it offers a perfect balance of power vs easy interface. I also tend to use stacks of synths within a Combinator with a submixer and effects as described above.
A final consideration may also boil down to CPU. If you have a slow computer, using multiple instances of Thor may not be viable! (Definately time to upgrade imo)
Check this link for a comparison of Subtractor vs Malstrom. --->
http://dubstepforum.com/viewtopic.php?t ... sc&start=0
Check this link for more opinion on Thor. --->
http://dubstepforum.com/viewtopic.php?t ... highlight=
Check this thread for more opinion on Reason as a program. --->
http://dubstepforum.com/viewtopic.php?t ... sc&start=0