Thanks Robin, although you over-explained lots of stuff (and sometimes you misunderstood me), the trick that made it work well in this particular PC was:

/usr/bin/jackd -R -P 70 -T -d alsa -n 3 -r 48000 -p 128 -d

I always used qjackctl... I prefer the mouse to the keyboard, I'm just trying to figure out what stuff in Qjackctl I configured different from that console command.
(I already had done the rest of the stuff - the linux-rt kernel, etc, etc - )