Thanks for the replies... I was only able to try things out saturday because I didn't have the phone at home...
It wasn't rooted yet, but that's done now. I flashed it with cwm recovery and Revolution HD 31.3.
During the installation, R-HD suggested the installation of PureXaudio - which I selected. There was still no change, the maximum volume of the phone is still very low.
So I checked the post for it, downloaded the mods pureXaudio and xLOUD to install them manually from recovery - still no change. I don't find anything in the settings either concerning those mods, so I don't really know what to do next. The post also states it's system wide, that nothing needs to be done for it to be effective...
Continuing my reading I came across some posts advising to modify some file in the system folder, but I can't retrieve that file. I tried by connecting the phone as media storage, I also browsed the phone's system folders with ES File Explorer - no luck.
I also checked the posts for viper4audio but I can't figure what I have to do exactly... I must say I read it rapidly 'cause I needed to go... but I do have the phone with me now, so I can try everything out.
pureXaudio seems to be a quality enhancer -- I should say, the quality is not the issue, it's really the volume.
Question : with those mods installed, shouldn't I be able to activate Beats Audio again, even without headphones ? Because I still can't...
I hope some of you can tell me if I did something wrong, or didn't do something I should have... installation of those mods didn't change a thing for me (apparently).
I'm about to flash with other ROMs to see if something changes - I'm open to all suggestions.
And as for AudioX - I suppose it was pureXaudio because I can't fine anything called AudioX in the Play Store or even by Googling it.