Edoardo, what follows might be obvious, but I've read some of your posts letting me think that it is not.
You seem to believe that some boxes that features some of the usual components of a synth will provide you with the sound of a sunth. If you think so, you might be disapointed. In others terms, it's not because the 104M or 103 have an LFO or the 107 has a VCO that they will make your guitar have the sound these particular components achieve in a synth.
Substractive synthesis in a nutshell…
Here is a quite simplified description of the audio signal flow:
- First component: the oscillators, incarned by VCOs in Moog synths. They are the initial source and produce a signal usually rich in harmonics.
- Second component: the filter, incarned in a Low Pass in the Moog instruments. It allows to remove some harmonics from the initially rich signal, the one produced by the oscilator(s).
- Third component: the amp, incarned by a VCA in Moog synths. It is usually associated with an envelope so that the volume can evolve in time.
Besides the audio signal, you can have other signals that control the different component described above. These signals are generated by different controllers (amongst which lie the LFOs). It's said that a controller (source) modulates a parameter (destination), that is it replaces your hand tweaking the knob for this parameter.
- An LFO allows to replaces your hand turning the knob periodically.
- An envelope is another way of having a parameter evolving in time, not periodically this time.
- The note you play on a keyboard will also modulate the pitch of some oscillator.
- ...
Now you have the basic concept. BTW, flangers, phasers or even chorus embed LFOs. They modulates delay times, just as in the 104M.
NOW!... what you seem to want is obtaining a synth-like sound with your guitar, your guitar being the source of the audio.
If you plug it into a Phatty, it will take the place of the oscillators, BUT NOT CONTROL THEM. That is, it will go through the filter and the amp without interacting with the component that produces that "square wave" you're after.
With the 107, you're closer to the solution: although the pitch of the VCO is still not controlled by the note you pluck, a feature called "sync" tend to simulate this. Still not what you are after though.
Here is what I think you need:
- Either an intrument/FX that allows you to control an oscillator with th pitch your guitar produces and the volume of it with the volume of the sound out of your guitar (BTW this is a MONOPHONIC process, most of the time: you won't be able to play more that one note). This is not very common. The Korg MS20 does it, then you can try software but it's not analogue.
- Or an FX that is able to process your guitar sound in order to transform it into a square wave. The usual method here, used in fuzz, is to over amplify the guitar signal until it is totally distorted: the chopped off waveform is very squary then. Ithink the MicroSynth is kind of working like that, with a bit more process to get a cleaner sound.
Voilà. I hope it helps. I hope it does not make things too complicated nor breaks your dream
You may be interested in reading some deeper substractive synthesis documentation, like online tutorials.
while the second offers too 
