Very interesting! I had similar thoughts of creating something like a virtual Quantum, with software.
I love VCV rack, but for such a task, i would tend to use Reaktor 6. Just because it allows the creation of a custom GUI. But you are absolutely correct, the computing power would be immense. Even if it was possible to create all the engines, the latency would probably way to height! It’s very possible that the quantum uses multiple cores. I have a few years of experience in programming. My theory is that Waldorfs programmer wrote all the important functions for the sound engines and manipulations in assembler language. The rest is probably a very optimised form of multi threading between the cores, like you already mentioned. (Virus Synthesizer, for example, are coded complete in assembly, for the same reason).
The GUI part is probably C/C++ code.
I even did some research into the VSTi documentation from Steinberg, but unfortunately i have zero experience in sound programming(I always used libraries for sound, in my own programs). But there are even applications that are basically VSTi Editors. I need to to more research, to decide if it would be possible, to do this with a editor. The reason why VSTi and not Reaktor would be the compatibility with DAWs and the fact that the complete GUI could be created from scratch, with multiple windows, and context related sub menus. With VCV and Softtube modular, you would need to re-patch everything if you wanted to use a different engine.
I tried to build a resonator with „audulus“ (I highly recommend this software, you may like it too). The resonator consists (in my case) of 16 bandpass filter, and multiple super short delays. You can send a simple click from a noise Source, or even envelopes, into the resonator and it results in a bell like sound, depending on the resonance and delay times etc. A little bit like a comb filter. But 16 filters are not enough four a good resonator. My resonator alone eats almost all the computer power!
But definitely a nice experiment.
I have NAVE on the iPad, would be interesting to connect the output to the input from VCV rack, and from there into a DAW. The cool thing on VCV are the (free) Mutable instrument modules. (I think the are called „audible instruments“). The module „Modal Synthesizer“ and „Resonator“ is basically what i kind of expected from the resonator of the Quantum. I‘m sure you already used those. Ironically since i use VCV, i want to start a eurorack even more, mainly with the „Mutable Instrument“ Modules XD
VCV has even a granular sampler modul, but i need to do some research, it kind of doesn’t what i want. Basically we have all the building blocks, but in different applications. If you want Quantum like sounds you could create a part in VCV, save the results and import them into a DAW sample player. From there the sounds could be played and processed further. Of course this makes only sens for some experiments. Then there is the thing with the standard Oscillators. The Quantum allows super-saw like wavetypes. Not only for the saw, but for all the other standard waveforms. Like super-sine, super-squares ect. I have no idea how I could achieve this without programming my own VSTi.
The complex Modulator would be easy, („massiv“ has a similar function for every LFO), EQ, Compression and the fx engines could be done by using the DAW plugins. Granular would be relatively easy to program, wavetables too. But that’s only in theory. If i would try to recreate Quantum in a VST plugin, i would probably need a few years and the endproduct would probably suck, uses all the cpu resources and the sound would not even be nearly as good as the Quantum. But it would be super cool to have a plugin that can be as flexible as the Quantum, and unlike the hardware, it could be upgraded with new features, filter models, multistage envelopes and other cool things. Plus, i would love to create my own GUI! It may sound strange but I have no idea how waldorf could build this thing for such a „low“ price. Of course it’s expensive but if you analyse the hardware, it’s kind of a awesome deal. A Eurorack case with those features would be gigantic in size and price!
Sorry for the long post!