XAKT is a relatively simple Spectral/Additive synth. Consisting of 2 -16 partial additive oscillators (the Red and Green Sliders) the morphing between those being controlled by Envelope or LFO.
The Filter section is rather unusual and was designed to further enhance the digital quality of additive synthesis. Either way the demo below shows why Xakt is popular with some.
Again this is a plugin with the older multicore bug so you may be limited to a single instance in a project.