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

MacBook Pro (USB 2)
Cambridge Audio DAC Magic Plus
Musical Fidelity A3.2 cr Dual Mono Power
B & W 804

I find that 24/96 material from Linn sounds great.
+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.
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 !
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:

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

Next to these, maybe the Music First.

Steve N.
Empirical Audio