OK, new theory. Perhaps I am seeing poorly written ALSA code in applications, trying to write to hw:0, bypassing my 'default' statements in asound.conf. Is there a way in asound.conf to reset device numbers to specific alphanumeric names? In my case, if I am understanding this correctly, to set hw:0 to point to hw:jack ?

J.E.B.

OK, perhaps a stupid question:  does jackd mix?  In other words, let's
say we have two or more audio applications outputting to Jack; will Jack
mix the streams?  Or does it send them each independently to the sound
driver?


Well, yes.  The mixing is done on a port-by-port basis.  Whenever
there is more than one input connected to a JACK port the
corresponding samples are added together.

This can produce float values greater than 1.0, which will overflow
when routed to a physical output device.  But, there are many JACK
applications that allow you to adjust output levels.