choose a DAC to complete my system

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.

Three things: 1) don't use the Oppo for a DAC, 2) eliminate, don't replace the active preamp. Active preamps like this are the bain of audio. If you are not willing to spend at least $10K on a really good tube pre, forget it. 3) make certain that your digital source has extremely low jitter. The computer alone will not do this. If you want the very best result, use an outboard USB converter with its own power supply, like the Off-Ramp 5.

There are a couple of ways to deal with the preamp:

1) use a DAC with a strong output driver (not an op-amp) and feed it to a passive transformer linestage, such as the Music First (I just happen to have a silver-wound one for sale). Output impedance of the DAC of <50 ohms is required. Avoid resistive passive stages as they create impedance problems downstream

2) select a DAC with a superior volume control technology, that beats all active and passive preamps like this one:
http://www.empiricalaudio.com/products/overdrive-dac. This one adds no components to the signal path, does not change the resistance or gain in the signal path and actually increases the S/N ratio and reduces the distortion as the volume is lowered. No other volume technology does this.

Do not use 100% software controlled volume.

Steve N.
Empirical Audio