Anakirob mentioned a few I haven't tried.
I was sequencing on Music-X 2 for several years. It's a midi sequencer with some simple sample editing and playback as well. The features are nothing special, but I liked the interface, as it was very intuitive for me.
Bars & Pipes seems to be the most popular midi sequencer for Amiga. I believe there is still a Yahoo group/forum dedicated to Bars & Pipes. I believe the complete program is available for free download now, and there may even be some modules/expansions being developed by some users (I read about this some time ago, but can't remember the details).
There was a nice sounding software 303 emulator which I tried years ago. It was using 8 bit sound, and had an inherently gritty and raw character (I think even more so than the h/w 303). However, it was only a sample generator, and not a real time synth, so I didn't use it much. Also can't remeber exactly what it was called.
I've never used Amigas as a sound source much, so can't comment on hardware.
Do you own a miggy now, or are you just testing the waters? What music are you working on?