Bit by bit here...
How does MI's Links compare to the 2hp mixer?
It doesn't. The unity-gain on the Links can mix...but you have zero control over the input levels unless they're attenuated from outside, which means yet another module would be needed. Part of the key to using a mixer is to make those little tweaks to the levels...either adding a little bit of a fast LFO to your pitch CV for a touch of vibrato, or backing down an audio level that's coming in too hot versus the other sources.
What are your experience with XAOC's Zadar as a quad EG - that would even double the number of available envelopes, wouldn't it?
Yes...BUT. The Zadar really works best with its expander, unlike the Batumi which can work well enough without its expander. If you can cram both in, though, that would make for a better solution. But the A-140-2 wound up in there because it physically fit; to get a Zadar and its expander in, you'd have to either expand the cab or lose something else.
Wavefolding: Is this a function that would be non-negotiable for my setup?
No, but it's worth noting that wavefolding and what the Zvex module does are similar processes. ANY sort of distortion affects the behavior of the incoming waveform. But this is key here: Zvex Lo-fi Junky= $299, Tiptop Fold = $135. The Zvex also doesn't allow mixing/crossfolding.
One other point: while the Zvex can do compression, there's not that much use for that in a modular unless you're either using it to...yep...waveshape via clipping OR you're using it like a typical compressor to control levels on an external signal. Unless you're talking about something brainshatteringly-expensive such as the Cwejman modular compressors, you're far better off using an EXTERNAL comp/limiter to control your modular's dynamics. You'll have more/better control that way...and, if you get a unit that has sidechain keying, you can send a modular trigger to that to "pump" the comp/limiter in rhythm with ease. You could even run that off of a trigger sequencer, making the entire modular work like a percussion instrument that way.
Also, did you come across Malekko's Quad VCA? If I'd get the Varigate-Voltage Block-Combo, it supposes to pair well with both
Sure. They're just large. You'd have to do some major reworking to fit those. However, I like the "variable curve" VCAs that Intellijel and Mutable both have, since you can alter the VCA's response on the fly to mess with dynamic range, CV/mod influence, etc.
Finally, I believe the 4ms Listen4 (Quarters) should work as both a stereo mixer AND Line Out. Am I right?
-- dance_a_little
Yes. However, the Listen Four offers no VCAs over any functions. It's purely manual. By using the Quad VCA as the primary mixer, though, you then can control the mixing via its VCAs, then use the Black Hole DSP as your "stereoizer" to widen the mono output from the Quad VCA. In fact, since you have CV over so many of the BHDSP's functions, you can actually make rather complex and constantly-shifting stereo imaging by sending modulation signals to its control inputs. This also saves space for expansion. Which gets me to...
This is not a large build. You have limited space here. It's not a good idea to do redundant modules in this sort of circumstance, because you'll quickly wind up in a situation where you have to leave something necessary out, or you'll need to go with a larger cab. And I see this problem all the time, where users are trying to cram ALL of their desired functions into a single small case. It doesn't work. Instead, you need to find function-dense modules that can cram lots of capability into a small footprint. But this ALSO has a diminishing return problem, as farkas notes above. Yeah, you could build the whole thing out of 6 hp and smaller modules...but it would totally SUCK to try and play an instrument that's laid out that tightly. Ultimately, there's a balance that has to be struck to make the build ergonomically suitable, otherwise you'll wind up with a boxful of uncontrollable and expensive nonsense.
mowse's "What is that thing behind you?" thread here in the forum gives a better solution. Instead of trying to smash an entire studio's functionality into a little case like this, you might want to think about their plan, which breaks up functions into different "zones" and even different cabs. This makes for a much easier work environment, because you know intuitively which part of the rig to turn to for the functionality you need. Yes, in the end this approach tends to cost more...but what you lose in $$$, you GAIN in usability, and that makes all the difference between a rig you'll work with for many years, and a pile of crap that's destined for eBay.