Hi,
Sorry for the late reply - work is v busy and I'm trying to upgrade the clock on my ST too.
You are right about the white caps - they form part of a feedback circuit and are directly affecting the analogue signal.
I would replace these with Relcap RTE but you might need to solder them in from the other side. I would be very careful about changing the values - you might seriously affect the negative feedback frequency bandwidth. If you look up the datasheet for the DAC, you'll probably find a suggested I/V circuit. Asus engineers seem to go by the book so it'll probably have been copied exactly, values and all. I think it is a PCM1792 ? If so, check page 35 of the datasheet here :
http://focus.ti.com/lit/ds/symlink/pcm1792a.pdf
In a perfect world, you would change the values in this circuit to match your op amps.
There are also some smt caps near the op amps in the I/V stage.
I am looking into replacing them but at this stage I haven't done any measurements to find out what values etc.
The other caps worth changing are the 220uF/270uF electrolytic caps - nine of them in all on the board - they are the power supply caps closest to the components.
Nichicon caps are renowned for being bright and detailed with high-bandwidth - and the card has this sound. Changing the electrolytics can have a surprising effect. It's not that the FG are bad but they have a signature and changing them can help to tune the card to your preferred tastes. I like SilmicII and Philips BC but everyone has their own favourites and finding ones that balance with the rest of the card is the real trick.
Have fun !
Tom
PS Here is a pic of my ST with a Vanguard clock. Still working on this...