Good, creative VSTi stuff :)
Sonata features two separate but similarly controlled generators, one for the sound's immediate attack and another for the fundamental tone. Both consist of rich harmonic spectra which are filtered to provide desired overtones. Each possesses its own exponential decay envelopes for amplitude and filter, and its own ADSR envelope.
Sonata produces sounds with consistent timbres in both high and low registers, allowing for better two-handed keyboard performance and ranged melodies. As a result, sometimes each note can get a little CPU-hungry.
ps. You can find additional soundBanks for Sonata at sohasound.com