no - it’s not normal for there to be distortion or glitches.
sorry, but Im getting totally confused about what problems you are having, since you keep moving between different topics - and you aren’t saying what’s going on very clearly.
a) the new OS image is required for USB audio interface to work.
and make sure you use the same sample rate on your computer and SSP.
use 48k sample rate on both.
b) try without any VSTs , just to eliminate them from the equation.
first scale down signal levels, to make sure you are not clipping.
Id personally try with a sine wave, and then reduce its level, so you know its not clipping at output stage.
if you are not using PMIX (since you don’t have any vsts installed!) then you will need to ensure that the levels are a bit level, just in case there is a DC offset.
try first with main outputs , not usb…
remember that eurorack levels are ‘hot’, you should attenuate them when feeding into mixers/audio interfaces that expect line level.
… you can do this in ‘software’ within the SSP, simply scale down the output signal…
at this stage there should be no distortion/clipping…
you should check the scopes on the output module of the SSP to check for clipping (which is why a sine wave is useful, since its easy to see)
so now, you should know if the output stage is ok, and also there is no internal clipping.
if that’s ok, then you can try USB. … this should ‘just work’
if not, then you there could be a problem on your computer side, just like any audio interface, if you have rogue processes, or things like wifi, specs of the machines -these can cause buffer under-runs which are going to sound like audio glitches.
Ive personally not experienced this on the Macs that I use when I use 48k sample rate.
I cannot remember what buffer sizes I set, probably something like 256 samples, but it depends on specs of your Mac.
specific note on polyphonic voices:
of course, when playing multiple voices , each new voice is summed, so you have to compensate.
that’s just dsp for you , so if you 4 voices, you will potentially need to divide each voice by 4 , so its only 25% of full gain. in practice this will likely be a bit quiet, but that’s the theory.
note: this is not quite how polyphonic synths work, they will tend to use a kind of compressor… this means that when you play 1 voice its about the same as playing 4 voices.
(again this is not SSP specific this is just the nature of dsp)
whilst the above may seem like a lot, its not really…
basically it comes down to just make sure the gain structure is correct…
what concerns me, is most of this you have to deal with eurorack generally, so, I suspect you have already looked at.