OSX: Plugin output of values +1.0 and -1.0 is silent. (Demo attached).

Yep, a loud-as-possible square wave at roughly 1 kHz. (At least there's no aliasing! ;) ) So you had system volume maxed, track and master volumes at *exactly* 0 dB, and the gain slider in the plugin itself at 0 dB? I'm trying some other hosts, I'm back to being not so sure now it's *just* the driver. But need to do a few more tests...

The plop thickens... It occurred to me that iTunes was playing the rendered file just fine, which should also be outputting nothing but -1.0 and 1.0 (unless it's applying some kind of gain, but I had it's volume maxxed.) So I downloaded the Ableton Live demo and my test plugin is sounding fine. As far as I can see my track and master are both at 0 dB so it should be the same as Reaper. This may be an Edirol UA-1000 with Reaper problem. Let me know what happens with your other Edirol driver for your FA-66. (Though I imagine they may be quite different as the UA-1000 is USB 2.0 and the FA-66 is Firewire.)

Hmmmm, Live was set to record to 16 bit *files*. But I don't think that affects the internal bit depth. (Which would normally be 32 or 64 bit floats anyway, not 24 bit ints like the audio driver takes. The audio path in a host is typically kept floating point until the very end, when it's passed to the driver.) Anyway, I changed it to 24 and it still worked fine with my test plugin. Same with Reaper. I see a setting under "Project Settings