I’m a bit of a modular synthesizer addict. A synth with a great sound is of course a wonderful thing, even if it has only a few knobs and switches — but I like nothing better than to connect signal sources to destinations in odd ways so as to create a sound that the instrument designers never anticipated. When I see a synth with one LFO, my reaction is, “Why aren’t there three?” When a modulation matrix has a measly six routings, I’m insulted. There ought to be at least 24.
Happily, the world of software-based musical instruments is well supplied with modulars. We’re living in a golden age of modular. Every company has its own ideas about design, so there’s no shortage of peculiarities to sort through, but there’s something for every taste.
I’m not going to try to do a full-on comparison of modular instruments here. That would require a book. (Oxford University Press agreed that I should write the book, but their offer of an advance against royalties was insultingly small, so I had to turn the deal down.) I’ll just offer a few random observations to whet your appetite or confuse the heck out of you.
One of the best software modulars is also, counter-intuitively, one of the least expensive. VCV Rack is mostly free, though there are some paid “premium” modules in the collection. The ecosystem of third-party developers is quite large, and some of the modules are quite exotic — and not always well documented, I hasten to add. Or documented at all. But the big downside of VCV is that at the moment it’s a stand-alone program. It won’t run as a VST plug-in in your DAW. An upgrade is scheduled for November 2021 to add that capability, but the new version is going to cost $99.
My experience with VCV upgrades is that when 0.6.x graduated to 1.0, a number of cool modules were left stranded by the side of the road. Those third-party developers were not all on board. As the months went by, most of them revised their code for 1.0, and because VCV is mostly an open-source community, if a developer abandons a cool module someone else may be motivated to pick it up. I expect the same time lag to happen with the release of 2.0, so I’ll certainly keep 1.1 on my hard drive when I upgrade.
Here’s a strange fact: VCV is natively 16-note polyphonic. Many of the recent entries in the modular sweepstakes (notably Softube Modular and the new Multiphonics CV-1 from AAS) are limited to monophonic operation. That is, you can set up multiple signal paths so as to play chords, but each oscillator or filter is going to produce a single musical line. The same thing is true of the Complex-1 module in Reason. Reason itself is of course polyphonic; or rather, most of the synth modules in it, either native or third-party, are polyphonic — yet the CV/gate implementation on Reason’s back panel is monophonic. Arrggh! The base installation of Cherry Audio Voltage Modular is strictly monophonic, but they do have some premium modules that introduce polyphony.
In this day and age, monophonic operation is a bit baffling as a design choice. Sure, there’s some user overhead in helping your users learn how to do modular polyphonically, but the advantages are huge. VCV is not only less expensive and equipped with a lot more cool modules, you can play chords. How do those other systems hope to compete? One of the earliest software modulars, Native Instruments Reaktor, is natively polyphonic, by the way, but developing your own patches in Reaktor is not an exercise for the faint of heart.
The advantages of any software modular over a hardware system are massive. You can save and load patches. If you own a given module you can instantiate it a dozen times in a single patch (not possible with hardware, obviously). The cost of software (ignoring the cost of the computer) is a tiny fraction of the cost of a hardware modular. And because developers don’t have to stock parts or pack their devices in shipping boxes, they can devote more attention to building odd but wonderful modules.
As one illustration of oddness from among dozens I could show you, here’s the Mog step sequencer in VCV Rack.

Doesn’t even look like a step sequencer, does it? Every step (that is, every hexagon) has two trigger inputs and four trigger outputs. Each hexagon steps alternately through its internal value and its external connections. Patterns of notes that are dozens of steps in length are all well within reach, though perhaps not so easy to envision in advance. It can play several lines at once, sending them all to a single oscillator, and the lines can interact with one another.
Let’s compare that to a step sequencer in Cherry Audio Voltage Modular:

Eight steps. The gate output can be switched on or off for each step. Bo-o-oring. Can it go backwards? Play steps at random? No and no. There’s nothing actually wrong with this module, except that it reveals a want of imagination. And imagination is what modular synthesis is all about.
Maybe I should write something about the insane world of VCV Rack’s envelope generators. Maybe I will. You want 16-step envelopes with conditional logic? There’s only one modular instrument that will do that (well, maybe two if you program your own envelope generator in Reaktor, but that would be a nightmare).
Down the rabbit hole. Would you like some more tea?