Hokay...let's get into this. Thus far, the equipment you have on hand makes sense, although using it in tandem with Reason where timing issues are critical might reveal some limitations. I've heard from a lot of professional types that Reason can have problems with timing and sync when dealing with external clocking. My best guess is that Reason varies its latency, so if you start with a fixed latency value and Reason opts to alter this on the fly without warning, your timings between in the box and outside of the box can slip and get a bit sloppy. I've also heard that FL can have issues of the same sort, but not to the magnitude of Reason. And yes, I've used Reason (quite some time back) along with several other DAWs over many years, and I eventually opted to stick with Ableton Live out of experience...which has largely been very positive. So if you notice timing 'slop' starting to ease into your work, suspect Reason first.
- The case : the TipTop Audio Mantis looks nice and not too expensive. I like the look of the Eowave 104HP studio case too. Any reasons why I should choose one over the other?
OK...with powered cases, you need to take several things into consideration. The most obvious is amperage...a sufficient supply should actually supply more than you need, because some modules can load-spike a bit on power-up, with some being worse than others. You want a lot of margin between your total module draw and the supply's rating...and not only because of this point, but also because letting the power supply 'loaf' will mean less component wear, and that spells better stability and reliability over the long-term. I personally like to overspec power supplies by 1/3rd, but that's an old habit from amateur radio work where amperage draws can sometimes fluctuate sizably and abruptly. +20%, however, is good enough.
Also, check what sort of supply you're dealing with. There's two kinds; in Eurorack, we almost always deal with switching-type supplies. These actually use high-frequency methods of regulating and rectifying incoming AC...and because of this, some of them can be noisy at the DC rail outputs. So if the case in question has a switching supply, you're going to want filtered power busboards...these not only knock down the HF crud from lesser-quality switchers, they also help to remove backplane crosstalk resulting from modules sending crud back down their DC supply lines and dirtying up the power rails. The other kind is linear supplies...the 'gold standard', really. Very clean and stable DC outputs, and usually beefy components are used in these that don't tend to fail from simply being a little touchy. Drawbacks: they're heavier, and they're more expensive. But those are really the only drawbacks; performance-wise, they're a big step up from switchers, and in big 5U systems, you tend to see them much more than switching supplies. Honestly, these days I've been looking a bit more seriously at Erica's powered 126 hp x 2 cabs; sure, they're 200 EUR more than the Eowave, but those Latvians build those things for serious work, with 2.5A on each 12V rail via an internal linear supply. And also, this is a good example of getting what you pay for...with hefty linear-supplied DC being well worth the price!
- I'd like to interface my current setup with the modules, hence the Pico INPUT to get the sounds of my synths eventually. Any opinions on this module? Will it keep the signal clean?
As noted above, Erica builds serious stuff. The sole flaw I see with the INPUT, though, is that it lacks either an envelope follower (to derive CV from the incoming signal's dynamics...very useful, actually) or a gate/trigger comparator (to send one of those when the incoming audio's dynamics cross a certain amplitude level...again, quite useful as a control source). But if the idea is to just send audio in without deriving any control functions, it'll work well for that.
- My audio interface is a Focusrite Scarlett 18i8 that should be able to handle modular levels. Do you advice nevertheless some kind of attenuator / mixer before plugging the outputs of the modular? If yes, why? :-)
Here's why: first of all, modular synths can be a bit touchy and unpredictable with signal levels...these can jump around a LOT, from very low, barely audible to (potentially without warning!) 10V RMS or more! If you want to be 100% certain that you can protect the Focusrite (which is a fairly low-end device, not something particularly rugged) from input overvoltage, you WILL need something to step the synth levels down to normal line-levels.
Second: the Focusrite probably has balanced inputs. The patchpoints within a modular are 99.999% UNbalanced (there's a couple of exceptions). These have different impedances, and depending on how touchy the Focusrite is about incoming impedance, it might not like something other than a 600 Ohm balanced incoming signal, resulting in several sorts of irritating outcomes like weird frequency responses, distortion, or even potentially component harm.
Third: ground loops. These are frickin' annoying hums and noises that creep into audio when a signal source is on one ground, but the device it's connected to is on another, and the ground plane tries to establish itself via your audio line. Avoiding this is done in two ways: ground lifts at the connected device (potentially less effective) and balanced and isolated outputs at the source (the 'correct' way). Now, you might argue that if the synth is connected via a DC line from a 'brick' supply, this won't be a problem; if so, you may be in for some annoying surprises, because the ground plane will ALSO try to establish itself via the DC rails under certain conditions, which will destabilize the synth altogether due to AC 'ripple' on the DC supply.
Last: DC offset. You DO NOT want this coming out of your outputs! DC, when fed to amplifiers and speakers in sufficiently high voltages, can cause everything from the infamous 'DC thump' (best known from the ARP 2600, which had DC-coupled outputs) when a signal is present to, under extreme cases, actual electronic damage to amplifiers and PHYSICAL damage to speakers. I've seen this, btw...it's a definite 'oh shit!!!' moment that you don't want to experience!
The solution to all of this is to have an output module that offers some sort of audio isolation, certainly DC isolation, and preferably balanced TRS or XLR output jacks. Best of all are isolator/outputs that offer transformer isolation, as having a little 'iron' in your signal creates very small euphonic harmonic emphases due to transformers' having hysteresis...which is a complex thing I'm not going to explain here (I don't have the time, space, or desire for carpal tunnel syndrome!), but which slightly emphasizes even harmonics in a way that 'warms' the audio and tightens it up without imposing apparent distortion. This, plus attenuation to get the synth-level signals under control, is 100% necessary at a modular synth's output. If you're serious about how your instrument can and should sound, something of this sort is absolutely essential!
- I plan first to get to the "basics" (VCO-VCF-VCA-Envelope...) with the Black Wavetable VCO and the Polivoks VCF, the Z4000 and the Veils. Any opinions on this?
See https://www.modulargrid.net/e/forum/posts/index/3015 I don't write these things for lulz, after all.
- Any good suggestions to fill the gaps are heartily welcomed :-) Still looking for some nice effects modules that could also be used with my actual synths.
Well, yeah...but it seems like you need to get to a more concrete stage in build development first. You're missing the Clouds yet assuming Mutable's replacement will be the same form factor (which wasn't the case with the Plaits, which replaced the Braids), and that's not the best way to proceed. Remember: this isn't a race...you're not trying to crash-build something for a gig this coming weekend. Consider carefully...and do some research. Look at others' racks and look at classic modulars to see why they were built the way they were, and with what. Back up first and build several 'sketches'...if one or two look promising, hone in on what makes them promising. And don't be afraid to screw up, because MG has a 'delete' function whereas physical devices and the costs associated with them definitely don't. Good luck!