Problems with HDMI output resolution using Xubuntu 14.04

Question

I got a problem using Xubuntu 14.04 LTS while connecting it to the HDMI port from a Samsung LN40R81. Straight to the point, the resolution is lower than the required (1366 x 768). This resolution option does not appear.

Besides, there is no xorg.conf file to make changes. And I have arandr installed, by the way.

My xrandr output on console goes like this:

   Screen 0: minimum 320 x 200, current 1366 x 768, maximum 32767 x 32767
   LVDS1 connected 1366x768+0+0 (normal left inverted right x axis y axis) 309mm x 174mm
      1366x768       60.0*+
      1360x768       59.8     60.0  
      1024x768       60.0  
      800x600        60.3     56.2  
      640x480        59.9  
   VGA1 disconnected (normal left inverted right x axis y axis)
      1656x900_60.00   59.9  
   HDMI1 connected 1280x720+0+0 (normal left inverted right x axis y axis) 160mm x 90mm
      1280x720       60.0*+   50.0     59.9  
      1920x1080i     60.1     50.0     60.0  
      720x576        50.0  
      720x480        60.0     59.9  
      640x480        60.0     59.9  
   DP1 disconnected (normal left inverted right x axis y axis)
   VIRTUAL1 disconnected (normal left inverted right x axis y axis)
     1368x768_60.00 (0xf2)   85.9MHz
           h: width  1368 start 1440 end 1584 total 1800 skew    0 clock   47.7KHz
           v: height  768 start  769 end  772 total  795           clock   60.0Hz

I just need to add the 1366x768 resolution, or at least the 1360x768, on which will make least difference for its usage.

Greetings.

EDIT Trying Volker Siegel's links, now appears this:

    X Error of failed request: BadName (named color or font does not exist)
    Major opcode of failed request: 140 (RANDR)
    Minor opcode of failed request: 16 (RRCreateMode)
    Serial number of failed request: 36
    Current serial number in output stream: 36

LAST EDIT (TEMPORARY ANSWER)

Well, I checked a little more about my screen and found a temporary solution. Why? It only works using extended screen, not mirroring them. When I mirror them, I cannot change both screen resolutions to the point I could observe 90% of all screen content on both monitors. So now I am using extended screen mode, because I can change both resolutions apart.

Still, if somebody could help me to add screen resolutions to my Xubuntu (or explain me why I can't), it will be fully apreciated.