choose a DAC to complete my system

what are you using as a DAC now for the PC?? if you are just running it direct from the PC soundcard to your preamp almost ANYTHING would sound better.

also if your PC is a secondary source (meaning the Oppo is your main choice) I would spend no where NEAR $3k on a DAC ... the AudioQuest Dragonfly or something in that price range would probably suffice

as for your pre, I do not see that as a "weak link" in your system

i'm using the Oppo BDP-105 with USB cable.
from oppo website:
"USB Asynchronous DAC – By bypassing the low fidelity, poor quality DAC of traditional computer soundcards, the BDP-105 turns any computer into a high performing multi-media source by converting digital audio to analog through the ESS Sabre32 Reference DAC. If your computer can play it, the BDP-105 can convert it to analog."

http://i40.tinypic.com/t85kb7.jpg

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.