The out is a Doepfer Mini Stereo Mixer A-138s. This feeds into a Yamaha MG10x external mixer, which...
-- eexee
OK, let's stop there for a bit. For starters, there's a big level mismatch going on, since the A-138s outputs signals at modular voltages (up to 10 Vrms) and the MG10x has a 0 dB input reference at .775 Vrms (consumer level, which means that the input pres are NOT up to pro standards). That's a pretty wide difference, and probably plays a part in what's going on.
Secondly, since you mention that the sound appeared after shifting to the Meanwell-powered cab, are you 100% sure that the grounding for the internal P/S is set up correctly? Given that patching more modules yields more noise, I suspect that you've got a grounding issue, since the patch cables will tend to pick up more crud as they increase in number. Mind you, this isn't an issue unless you've got your AC supply going directly to the Meanwell. Also, by "grounding" I also mean that the patch panel itself cannot "float" relative to the internal DC ground, so double-checking the internal DC ground connections might yield a fault.
Third, if these don't work, the next suspicion would be a ground loop between the modular and some other device, particularly something digital given the character of the noise. Or it's also possible that what you're hearing is good ol' 60 Hz with a considerable amount of distortion from the level mismatch. You can test this by using a "ground lift adapter" if the Meanwell has a 3-prong AC input. Put that on the power cord, plug it back in, and if the buzz disappears, you DEFINITELY have a ground loop issue.
Another way to fix ground loops...and one I strongly urge if you're not living/working where balanced AC power is the norm (Europe, for the most part, has balanced power)...would be an isolation module that would go between the mixer's outputs and the inputs for the mixer. This also has the plus of these having level controls; something like Happy Nerding's transformer-isolated Isolator also gives you a ganged stereo level control to bring the level mismatch under control.
Lastly...and also a ground loop issue consideration...would be to "star-ground" ALL of your gear to a common ground point with some basic hookup wire. And even if you don't have an issue with this, it's still a good idea as it'll help lower your overall noise floor by draining any e-crud to that common ground, where it can then go right onto the ground leg of the AC. I should note that I do this in my studio even if there's no apparent noise/ground issues, as it has some very beneficial results that can wring out a bit more S/N, cleaning up the lowest digital levels.