Just a quick side note to all this... in learning more about JACK and LADSPA, I realized that I finally had a solution to an issue that's been a minor irritant for a long time: how to get a global graphical EQ in Linux. My headphones are somewhat lacking in bass; OTOH my speakers are lacking in treble and have somewhat "boomy" bass. I've been dealing with this by loading different EQ settings in Audacious (the music player I use when I'm on the computer) every time I switch back and forth between speakers and headphones, and just living with everything else sounding slightly tinny or boomy (depending on whether I'm listening on headphones or speakers).
Well, it turns out you can insert JACK into the signal chain between
PulseAudio and ALSA. PulseAudio becomes just another JACK client. Then you can set up equalizer settings (using jack-rack) that get applied to all of your audio. I've even got jack-rack configured with different EQ settings for the front and rear soundcard outputs; now I can just leave the headphones plugged into the front output, and the speakers (they're only 2+1) plugged into the rear output (I've got JACK configured to route the "front" audio signal to both the front and rear outputs, but through different EQs for each). So I don't need to plug/unplug the headphones *or* futz with EQ settings any more. Cool!
Setting things up like this also sidesteps the issue of JACK and PulseAudio fighting with each other, and keeps Google Chrome happy (previously, Chrome was getting confused whenever I started/stopped JACK; I don't think it likes when PulseAudio disappears out from under it).
To do all of this, JACK appears to be consuming only 5% of one core on a quad-core Phenom II. I can live with that.
Now I just need to figure out how to get it to come up configured like this automatically after a reboot, instead of needing to manually load up all the JACK tools and make the various virtual audio path connections. JACK appears to be very scriptable, so this should be a relatively easy problem to solve.Edit
: JACK in action (click the image for a full-size uncompressed PNG):