Thanks for all the help!
I tried eDuke32 and it´s fantastic! Very high resolutions, posibility of unfiltered textures, lightning options... And there is an especific option in the menu that solves my problem: Display Setup -> Color Correction -> Visibility. Changing Visibility to the max number, now the game does not have that excessive darkness in the backgrounds, and maintain the brightness, saturation etc.. of your liking.
GZDoom is also fantastic. Even more graphical options! I finally found the option to have unfiltered textures: Display options -> OpenGl Options -> Texture Options -> High Quality Resize Mode. Setting it to Off, there will be no texture filtering in-game or in the menus. It looks fantastic at 1680x1050.
And very nice the "Quake Classic" for QuakeHD (quakespasm is the name of that particular source mod, right?)
I have the same darkness problem with Quake II, Doom II and Blood. Are there source ports for these too?
Quake II also has source ports available. I use one called Berserker that makes things looks as modern as possible.
Other Quake 2 source ports you might prefer :
Yamagi (I haven't tried it): http://www.yamagi.org/quake2
KMQuake 2 http://www.markshan.com/knightmare
And some other Quake 2 source ports:
For Doom II, use GZDoom. GZDoom works for Doom and Doom II (and Heretic and Hexen). You just need to put the Doom 2 wads in the same directory where GZDoom is installed.
Blood's source code was never released. Without the source code, no source port. There is BloodCM, but it's incomplete and more of a re-imagining of blood, not a source port: http://m210.duke4.net
For Blood, I'd recommend a 3rd party program to adjust the gamma while in game (I haven't tried this program): http://www.majorgeeks.com/files/details/gamma_panel.html
I found that in this post: http://www.gog.com/forum/blood_series/blood_2_too_dark