Hello!
Synthesis was not so popular on the Amiga as it was on C-64 since sample playback capabilities (far better sounds at that time).
But you can check the old Aural Synthetica, released for free by Blachford Technology:
http://www.blachford.info/blachtech/It can't play sounds through MIDI, but you can save your sounds in 8SVX, Wav, AIFF format for loading them into your preferred program.
AFAIK it's a synthesis program that implements algorhythms through processor, not using special Paula hardware (which is not).
BY!