Tube and 'non-tube' equalization

Hi Folks,
 
What is the difference between Tube Equalization and 'non tube' equalization? I am asking because when I play around with the tube equalization in VX-64 Vocal Strip, I notice that the sound has a bit of a muffled feel to it, whereas if the equalization were to be done in the sonitus fx equalizer plugin, it has a more open feel to it. I hope somebody can explain. Thanks.
 
regards
glisando