Sound selection is probably the hardest part of making professional beats sound clean and loud!
You must think in terms of octaves (pitch), timing (where you place sounds), and timbre (how rich in frequency a sound is).
I do not think this is an issue about what Virtual Instrument you are using like FLEX by Image-Line or Nexus by reFX..
A couple other things to be aware of is the amount of effects sound designers put on their presets. They do this to make the instrument sound good, but when it comes to mixing and mastering, it takes up A LOT of space in our limited frequency spectrum!
What I’d say Alberto is email me your .flp so I can take a look.
Here’s how to prepare a FL Studio Project to email:
Create a folder
Export the .mp3 of the song so I can listen
Save the .flp in the folder
Save the sounds in the folder with File -> Export -> Project data files
(I hope I have the same plugins as you so I can open it up.. if not I’ll have the .mp3 you include).