You say in your layout description you have 'MIDI in the bottom left' but you have the ES-8 which is an Audio Interface, I think. You might be thinking of the FH-2 instead yes? That does MIDI to CV conversion so you can play from external USB devices and your DAW, etc. An interesting alternative to the Transmit 2 output is 4ms' Listen i/o. It's the same 6HP but has two mono inputs and stereo outputs, and both outputs can be used as headphone jacks. I play sounds out of my DAW, iPad, iPhone, acoustic instruments, (using the headphone output on my devices with a 3.5mm stereo-to-split-mono cable) through modular and then record them back into the DAW (with a stereo-3.5mm-to-dual-mono-6.5mm cable) using the line input on my audio interface. (I have it routed properly in my settings so there is no feedback issues). It works!
None of these need a buffered mult - a passive will be more than adequate
Don't worry, it's just an example, haha. He's already got two buffered multiples in the rack, I'm just patching his design. BUT... He listed his need to connect with 'external gear' and be 'a multi-voiced instrument'. So if he is pitch tracking from his DAW or keyboard split four ways by a passive multi into that Quad VCO and the voices sound out of tune—knowing full well he spent 13K on a rack and then cheaped out on passives just to save an extra $30 will make a dreadfully bitter jam to dance to.
That Music Modular Thing Four Big Knobs is a DIY project so you're gonna have to build it yourself or maybe find someone selling it already. If you don't have the soldering stuff you might want something else. I think you should check out Intellijel's Tetrapad as a potential substitute.
Anyways good luck! Hope you find your sound.