Hi,
OK, I spent a little more time using the new Undercurrent Extension, given I don't use Omnisphere that much these days, well.. I switched to Stack Mode, and that's when I began really hearing the magic, layering is the key here. imho. Creating custom Stacked instruments is where Omnisphere shines. WOW !
I started with 2 stacked instruments, each instrument can have up to four layers, so you can start tweaking the hell out of these two instruments in the Stack, change the source sample, synthesis, envelopes, effects, modulations, then proceed by adding a third instrument to the stack, and even limit it to be triggered at i.e higher velocity values, ..etc The creative options are endless, that's how I would work with Omnisphere.
You can limit the note range of a stacked inst. , velocity range it will play at, or even trigger stacks via CC# value. Programming the Modulations carefully, and creatively can make a huge difference as well to add motion, and perform in a musically expressive manner.
Cheers,
Muziksculp