The ESP Guitar Company

Well, your problem could come from a bunch of places.  What's causing the lack of sustain?  Is the string fretting out or buzzing?  Is the wiring not making great contact, causing the signal to cut in and out?  Is the hardware making good contact at the body and strings?  Are the strings catching in the nut slots?

I'd make sure that your bridge and stop-bar are seated well.  If you can, screw the stop-bar all the way down.  If you're already there, taking the guitar in for a good setup and inspection might help determine where any issue is.

Though all that being said, I think most people's thoughts of "not enough sustain" are just in their head.

Also if you're using a noise suppression pedal or multi-effect with a noise suppressor, you might need a different setting for the MX than the Gibson.  The MX might just have a lower volume output than the Gibson.  If you're testing your sustain through something with a suppressor (or noise gate), try turning that off and comparing the two guitars again.

Hello,

I'm not using any pedals at all. I'm actually thinking this could be related to external interference, because in certain times of the day, specially at night, I get a lot of noise. And if I point the guitar pickups facing the ceiling Por example, the noise goes away. And my amp in grounded.

The strings are not buzzing and the bridge and stop bar seems to be correctly adjusted.

There's a possibility of wrong wiring.., perhaps one of the pick up wires is bad, connected wrong or something?