Have you tried Synth Rack? Does it still behave the same?
I've never put soft synths in the FX bin so I don't know if there's any difference.

 Hi Bob, Scook
the problem is solved, in as much as I am happy to turn the engine back on.
To answer the questions about what I was doing:
I have a midi track, lets call it piano. It's output is hooked to an audio track which has TTS-1 loaded in it's FX box.
when I double click the TTS-1 icon in the audio track's FX box the TTS-1 window pops up, whereby I can change the instruments on particular midi channels or sample the sounds of the instrument on any particular channels.
BUT.....when I do the double click to get TTS-1 window up, the audio engine sometimes drops out. This I didn't notice to begin with. The upshot is that the preview buttons don't generate any sound and any keyboard connected to the midi channel doesn't sound either...This is fundamental really but I didn't notice the audio engine dropping out so I was stumped.
problem solved, I can turn the engine back on when I notice it now.
mea culpa!
thanks
Ed

Do not use the FX bin for that - load each soft synth into the synth rack, and simply route to and from it with an audio and a midi track.

Double-clicking the synth in the rack will open up the properties and let you change sounds and such.

Bob Bone