I do my music making with Ableton rather than modulars, but this is a topic near and dear to my heart.
When we talk about “music theory,” that might mean one of several different things. In most college and high school music classes, music theory is a formalized description of the harmonic preferences of Western European aristocrats in the eighteenth and early nineteenth centuries. If you take this description as a rule set, you can produce music that follows Western European conventions of that era, which to contemporary people sounds pleasant, but boring and stodgy. These conventions will probably not to be very useful to you in producing loop-based music, because they presume that you’re working with linear structures of tension and resolution, and the whole point of current electronic music is to avoid those structures. It’s not a complete waste of your time to learn Euroclassical theory, but it’s not adequate for producer purposes.
The more progressive schools also offer jazz theory, which is a formalized description of the harmonic preferences of African-American musicians and their white imitators in the United States in the twentieth century. Jazz theory is a superset of late nineteenth and early twentieth century classical practice, together with the blues, which is not well explained by Western tonal theory. Jazz theory is significantly more useful than classical theory to producers of electronic music, because concepts like chord extensions, unresolved dissonances, fourths chords and blues tonality all apply very well. However, jazz theory is mostly concerned with using V-I cadences to establish key centers, and in electronic music you rarely find cadences at all. (Instead you establish key centers through repetition, or there is no unambiguous key center.) Learning jazz theory will get you closer to where you want to be, but it will still leave some holes in your understanding.
Finally, there’s music theory as practiced by current music theorists, especially the more progressive ones who come from “new musicology” or ethnomusicology backgrounds. The idea in current practice is not to try to find universal rule systems that explain everything, the way that classical theorists did in past centuries. Instead, you look at existing musical practice in a particular style or genre, and see what patterns or rules you can derive. Sometimes you might try to use an existing theoretical structure to do your explaining, and sometimes you invent new structures. These people are doing a more formal version of what self-taught bedroom producers do: listen to a lot of real-world music and try to figure it out. This is the kind of music theory that is most useful for people on this thread. Unfortunately, it’s also hard to get your arms around it, because it’s being published in academic journals that are heavy on jargon. The good news is that all the younger and more progressive music theorists are on Twitter, and will happily answer your questions (including me.)
Here are some recommended resources:
Music Theory Online is an open-access online journal that publishes some excellent pop music analyses. http://mtosmt.org/
My favorite single book on music theory is Everyday Tonality by Philip Tagg, which encompasses a huge range of pop and vernacular music. It’s dense, but it’s worth the effort. http://www.ethanhein.com/wp/2017/philip-taggs-everyday-tonality/
The blues is the most important harmonic development in the past 150 years, and it is very poorly explained by most music theorists. I did my best to explain it here: http://www.ethanhein.com/wp/2014/blues-tonality/
I wrote a quick and dirty guide to some commonly used scales and modes here: http://www.ethanhein.com/wp/2010/scales-and-emotions/
And a guide to making chords from those scales and modes here: http://www.ethanhein.com/wp/2015/making-chords-from-scales/
My favorite books on jazz theory are The Jazz Theory Book by Mark Levine and Something Borrowed Something Blue by Andy Jaffe.
Finally, if you like my approach to music theory, I created some online courses with Soundfly that you might find useful. https://soundfly.com/courses/the-creative-power-of-advanced-harmony