Maybe the problem (with its solution) that I bring you below for some users was obvious, or they knew it, but I just found out and that is why I share it with you in case someone was in the same situation.
The problem is that since I use KDE always struck me that the equalizer of Amarok was disabled. As generally I have always used CWell, I hadn't paid attention to this matter until yesterday it started bothering me.
KDE uses a framework for Audio and Video devices called Phonon, and when we install ArchLinux with KDE, it always asks us which engine to use for said framework.
By default comes phonon-vlc and unfortunately, that engine does not allow Amarok use equalizer. The solution? Well, it is evident: Install phonon-gstreamer.
$ sudo pacman -S phonon-gstreamer
In the case of Debian must be:
$ sudo aptitude install phonon-backend-gstreamer
When installing this package in Debian with KDE 4.8, it automatically got GStreamer as Default Engine in System Preferences »Multimedia» Phonon »Engine.
But in the case of ArchLinux with KDE 4.11.2 I had to manually default it to System Preferences »Multimedia» Audio and Video Preferences »Engine, log out and log in again.
From that moment, Amarok I added a Audio Analyzer, effects of Fading and of course the option to use the Equalizer.
I leave you a small video that I made with three reasons: First, to test the groups in Vimeo for the project @ 10inFromLinux. Second, to see how it works. Third, to see how the videos look on this platform, so any ideas, suggestions or criticisms will be welcome 😛