You've just been lucky...inasmuch as the Morgasmatron has that input gain control and the passive mixer attenuated the output signal. Here's what happens when you feed line-level directly into a modular without a input preamp or some other sort of gain control...line level is actually extremely low when compared to synth levels. Typical line levels are .775V (-10) or 1.2V (+4) (although the actual peak-to-peak levels are different due to a bunch of impedence matching, math, and other forms of voodoo), whereas synth audio signals within a modular system can run at 8 to 10V peak-to-peak. So, without some sort of input preamp, the incoming line level signal is going to be far lower in amplitude than everything else going on.
Now, feeding a line level input with a synth directly...that's a little bit different. The most likely thing you'll run into is a majorly overloaded input on your line input device. Some devices, though, actually have enough headroom to deal with the higher synth levels and you can just attenuate as needed on those. But every once in a while, you'll run into something that really DOES NOT like anything too much hotter than a professional +4 dB line signal. Depending on what that device might be, you'll get a result ranging from massive distorted overloading to component-go-POP, which isn't fun. These days, there's not many things out there that can go pop when overloaded, but that doesn't mean they don't exist. Case in point: we were running a guitar through an overdrive into my Biamp MR/140 spring on a session many years ago. Sounded AWESOME...for a couple of minutes. Then it didn't sound like anything. Turned out the spring transducer couldn't handle that much drive and it gave out, and that was that until I could get it serviced.
So, yeah...the best thing to do is to be ready for incoming and outgoing line levels by having some sort of input preamp (preferably with an envelope follower...very useful combo) for line-level inputs, and output modules (preferably with isolation and balanced line outs to quash possible noise/hum issues before they start) to step your synth level signals down to where they belong.