Hi Adrian,
amsynth cannot be used with jack.This cannot be true. I just tried it on two different machines
To clarify, amsynth works with original JACK but not JACK 2, a.k.a jackdmp. This is because amsynth's JACK connection code is a little non-standard, and needs a rewrite to bring it up to date. I've been in contact with the upstream author, but I don't think he has time to rewrite it at the moment.
Cheers! Daniel