Advice

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

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?

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.

Look at your room . bass traps have never had a negative impact on any system i have ever heard . If you are a diy type at all they are simple to build . And it will save you at least half off of new.Search superchunk bass trap if you are at all interested in learning about them. In my experience some room treatment has had a bigger impact than switching amps and pre's in the same price range . Good luck !

+1 to what Zd542 said. If it 'sounds great', walk away and spend your money elsewhere.

The law of diminishing returns hits early and hard in this hobby. You could spend a fortune chasing the ever elusive 'absolute sound', or you could just be happy. The choice is yours.....choose wisely.

Step by step is the best way to go. As suggested, first ditch the Mac DAC. USB 3.0 outboard DAC's are in the pipe line and that will make a difference. It will also give you a path to further improvement.

A lot better IMO. If you reduced the jitter of the source by using a quality USB converter and a good power supply. Also you could also eliminate a ground-loop by adding a good 1:1 signal transformer between the DAC and the integrated amp.

Beyond that, you would have to change out the DAC.

Steve N.
Empirical Audio

"I have a modest system and it sounds great. Looking for some calibration, i.e. how much better could it get?"

My advice would be to leave it alone. If you are really happy with your system, why fool around with it? You could just as easily make it sound worse than better.

Calibration? (From The Princess Bride) "I don't think that means what you think it means." In any case, you seem to have chosen some nice components within that price range and you're probably not going to better them without spending a lot more. Look into some good power conditioning. I prefer Audience, but there are others. I know - Do I have to spend more on my conditioner than my DAC? Yes, if you don't want to screw things up.

http://www.needledoctor.com/Audience-adeptResponse-aR2p-Power-Conditioner

Well a lot...but usually a price not commensurate with performance. What's your room like? Where are you speakers placed? Are you looking to spend money or keep this simple?