choose a DAC to complete my system

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

After you fix the jitter, the preamp and the DAC, then you should focus on the source.

There are three paths, all driving USB out to the USB converter:

1) PC using optimized hardware (SSD, memory, power supply). Playback using Jriver (and maybe Jplay). Rip CD's with dbpoweramp

2) Oct 2009 Mac mini with optimized hardware (SSD, memory, power supply). Playback using Amarra. Rip CD's using XLD. The Mini can be bought on ebay for less than $400.

3) Server such as the Antipodes, which will drive USB out. This one is particularly good because it sounds as good with FLAC as with wav files. We used one of these with great results at the last RMAF.

Steve N.
Empirical Audio

"The reason that I mentioned trying a different cable is that every time I put a silver cable between an amp and preamp, I don't like it."

That is because you are buying the wrong cables. The silver metallurgy and care in assembly is critical. Most silver cables have broken crystal lattice, so they sound harsh. I used to manufacture and sell silver cables.

Steve N.
Empirical Audio