choose a DAC to complete my system

i add a pic of the system:
http://i40.tinypic.com/t85kb7.jpg

Pelo911,
the DAC is not your problem. Your cables have good reputation

If you are using a generic off the shelf, un-optimized PC then your sound will be incredibly mediocre for digital playback

Options
1) if computer savvy do the reading on computer audiophile site on how to build an optimized media server and follow through
2) if not your cup of tea buy an already built media server a la CAPS models

this will insure that you have mitigated to a high degree the multitasking contention and electrical noisiness of a commercial PC that holds you back from the better than CD/DVD performance you can get from a media PC via asynch USB to your Oppo player

Like others have said, we need to know what you don't like about your system. If you just want to try something, put a decent copper cable in between your amp and preamp. Its just a guess, but you may like what you hear.

Do you play CDs through the Oppo? How does that sound compared to using the PC?

do you mean to replace the acoustic zen silver reference ii XLR interconnect with other cable?

it's hard for me to describe (bad English)

almost no Bass , no soundstage
sound not full, not extended at the ends, not warm

i listened at CES to Focal Diablo and 1038 like i have
and i love it so much.