Advice

Nice system. My advice would be to try a new DAC, the Cambridge is good but there are certainly better.

Many of which you can try risk free in your home.

I second Steve's suggestion. Even though the DAC supports async USB, it's worth trying out a top-notch USB/SPDIF convertor (on trial?) to see if it betters quality. If not, then the investment is worth going towards a DAC upgrade. In my experience power conditioning is a must on your source. Even a Tripp Lite isolation transformer makes a difference.

Steve, can you recommend any signal transformer to go between the DAC/amp?

Nk - The best transformer on the planet is the Finemet from Japan. I use these in my Final Drive buffer:

http://www.audiocircle.com/index.php?topic=108715.0

You can find these for DACs on ebay. They are about $1K

Next to these, maybe the Music First.

Steve N.
Empirical Audio