Ok, now i got it:

I think i found out why you aren't supposed to use jack as root, it is quite unstable.

It turns out i disabled cadence from "session start applications" and now, the very first path i choose once i start a session is my final option, lol. If i open "sudo qjackctl" i get to start a root jack server with no problem, then i can "sudo cadence" to open my alsa bridge, and i get a perfect root+jack config. I can run root apps with jack as an output, and "pirata" (my user) apps with alsa. To be noted i cannot start a non-sudo jack server until i restart my system.

If once i start my session i open qjackctl without sudo i get a working "non-root" jack server.

I was having some realtime issues, until i noted "qjackctl" realtime configs are read over limit.d configs, so be sure to check that! Setting Realtime Priority to 70 on qjackctl fixed my realtime issues.

I don't think i have any issue at all right now.

Never thought i'd be saying that

Dear EEight:

I think (but i'm not really sure here) thefollowing did the trick:
usermod -a -G audio root

It seems jack was using an "audio" group for limits.d/audio.conf, and by putting "root" into it (and starting qjackctl with super-user priorities) i don't seem to get any errors of this kind.

Thank you!

I was still stuck with using pure data as a midi out, but it turns out i was mistakingly configuring "ctlout" on my puredata scheme, so i was adding info to the wrong "inlet". Basically, i set my values to the third inlet, when i should use the first for input.

Now Mixxx is able to read my potentiometer as an input device!

Thank you, mister!

EDIT: Hmmm, it seem i need to use the referred command every-time i boot ubuntu-studio. Is there a workaround?

@EEight Trying with "sudo" before the commands i get the same errors.
It seems to boot fine without sudo (even without your tips), but it's useless since i cannot change basically anything on pd-extended

I haven't really thought about it, but i cannot connect properly to jack with applications (Ex. mixxx or Audacious) if i launch the softwares on "super-user" (sudo) privileges.
I could connect to jack on pd-extended without sudo (without being root), but i wouldn't be able to do anything on the program.

I think what i need is to connect properly to jack with root. Is that possible?

Update: added "root" user to "audio" group. (I've done this to my personal user already)
No sign of success :/

Dear friends:

I'm opening this thread, after trying every single bit of information i could find over the internet to solve my problem. Lost nights n' nights trying to get better results, but i'm basically stuck on the same spot.

I'm trying to get my pure-data/jack setup on Ubuntu-Studio 14.04 RT kernel.
I've installed every package of KXStudio, and managed to get quite a good audio configuration:

As i read in many forums that pulseaudio is a common source of issues, I've got rid of pulseaudio, it isn't installed at synaptics, nor running at all.

I'm using qjackctl to configure my jack server, and i'm pretty sure everything is fine, since i'm able to configure jack as the adudio output in softwares such as mixxx and audacious, and i'm also able to use non-configurable (at least for me) software such as Firefox with alsa->jack configuration, all at the same time. Magic for my eyes n' ears

My problems start exactly when i try to use jack as the audio output for pd-extended (same error for regular pd, as a matter of fact).

To add a short background story, i'm trying (with quite sucess so far) to use pd-extended to get the some analog sensors (so far, i'm testing a potentiometer) plugged as analog inputs on my arduino-mega, connect my Arduino-Mega via StandardFirmata+pduino to pd-extended and use Jack to provide a Midi output to make "ctlout 1" object to connect with midi-input-able softwares such as Mixxx, and use it as a midi controller.
As of Pd-extended configuration, i'm doing quite nice, pd-extended is able to read my potentiometer values, which are scalled from 0-127 and are linked to a "ctlout 1" object.

The problem is, i'm not able to select Jack as an Audio Output on pd-extended!

After my jackd server is running (with qjackctl) and it is veryfied to work with the software/methods listed above, i open a terminal prompt and try "sudo pdextended" (i need super-user privileges so i don't get "Permission denied" errors):

The results are as such:

[1]On Pd-extended terminal:
JACK: unable to connect to JACK server
JACK: server returned status 17

[2]On the terminal screen i used to call "sudo pd extended":
user@PC:~$ sudo pd-extended
priority 6 scheduling enabled.
priority 8 scheduling enabled.
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
watchdog: signaling pd...
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
jack server is not running or cannot be started
open: /etc/pd/gem.conf: No such file or directory
open: /home/pirata/.pd/gem.conf: No such file or directory
open: ./gem.conf: No such file or directory

[3]On qjackctl "Messages" screen i get as follow:
Tue Dec 2 16:55:15 2014: JACK server starting in non-realtime mode
Tue Dec 2 16:55:15 2014: self-connect-mode is "Don't restrict self connect requests"
Tue Dec 2 16:55:15 2014: ERROR: cannot register object path "/org/freedesktop/ReserveDevice1/Audio0": A handler is already registered for /org/freedesktop/ReserveDevice1/Audio0
Tue Dec 2 16:55:15 2014: ERROR: Failed to acquire device name : Audio0 error : A handler is already registered for /org/freedesktop/ReserveDevice1/Audio0
Tue Dec 2 16:55:15 2014: ERROR: Audio device hw:PCH cannot be acquired...
Tue Dec 2 16:55:15 2014: ERROR: Cannot initialize driver
Tue Dec 2 16:55:15 2014: ERROR: JackServer::Open failed with -1
Tue Dec 2 16:55:15 2014: ERROR: Failed to open server

It seems to me there it's something related to wrong qjackctl settings, but if that's the case, why is it only related to pd-extended?

I'm adding Screenshots of my Qjack Settings

Since i cannot take a print-screen of Input/Output devices, that's my list ATM:
hw:PCH - HD Intel PCH (hw:0)
hw:PCH,0 - CX20590 Analog (hw:0.,0) #thats my mic
hw:Loopback - Loopback (hw:1)
hw:Loopback,0 - Loopback PCM (hw:1,0)
hw:Loopback,1 - Loopback PCM (hw:1,1)
(default)

If any useful info is missing, please ask me for it, i'm REALLY into fixing this.

Thank you very much,

Miguel.