Next upgrade?

Dear all,

In the pursue of the best possible sound - given a limited budget! - I am constantly thinking about the next upgrade (or mod) to my system. I’m sure I am not alone...

Right now I have the following :

- Monitor Audio Golden Reference 60 Speakers

- Primaluna Dialogue Premium Integrated Amp

- Cambridge Audio CXC CD Transport

- Border Patrol SE-i DAC

I like the system a lot, but I wonder what can I do next (excluding cables or power supply). What would be the place where I can get the most improvement from an (second hand) upgrade?

Thanks a lot for your suggestions!

Fab