Hi Adrian,
No problems over here with plain jack2-1.9.2. Works out of the box. Same is true for amsynth: works like a charm with jack2. Are you sure your jack2 package is in good shape? ;)
Possibly an update to 1.9.2 would fix this, perhaps upstream have put in some backwards compatibility options. I know amsynth has been broken on 64 Studio since we switched to jackdmp some time ago.
Cheers! Daniel