Hi everyone again!
I have done just what Fleib says. I have construct 8 pairs of RCA connectors, with resistances as parallel load with Y adapters for the two channels of my preamp.
The results dissapointed me.
The most "closer" sound for my taste was with 76 Ohms & 93 Ohms plugs.
Comparing with the previous situation (directly connected to my preamp) there was a better balance in high frequencies and a litle bit more bass. The middles become worse and nothing was comparable with my other turntable combination (Lenco L-75 with Denon DL-160).
I suppose that Technics SL-1200 MK2 with Denon DL-304 combination, was not make any better results from the other turntable, on the contrary... it was MUCH worse!
So.. after my last attempt to change the sound, the conclusion is that i must change the Denon DL-304, because i don't have any possibility for better matching, with my Onkyo's P-304 preamplifier MC phono stage...