Can I use Sphere with Universal Audio’s Unison preamp modeling?

Yes, definitely. While we generally recommend that the Sphere plug-in comes first in the effects signal chain, good results can be achieved with the Unison plug-in placed ahead of Sphere and in a Unison insert slot, as long as exactly the same settings are applied to both channels and the preamp modeling is not being overloaded. On the other hand, if you want to use a preamp modeling plug-in to add obvious saturation then it's advisable to move the preamp plug-in out of the Unison insert to after the Sphere plug-in.