If you like the basic character of the sound of your Cambridge 540, then consider the new highly reviewed Cambridge 840C. It has balanced out, so you can run it to the balanced inputs on your amp. In my experience, running balanced can make an improvement but a subtle one, not night and day.
I have owned both the Apollo and 840C. Both offer excellent sound for the money. The Cambridge, compared to the Apollo, is more up-front, and has a big, solid low end. I slightly preferred the sound of the Apollo (a little more relaxed than the Cambridge), but I found the Cambridge more user-friendly than the Apollo.
Here's one more option for you to consider -- keep your Cambridge 540 and get a good DAC. I really enjoy the sound of my Benchmark DAC -- in my system, better than both the Rega Apollo and the Cambridge 840C. You can order the Benchmark DAC-1 from Benchmark on a 30-day no-risk trial -- money back if you don't like it. The Benchmark has both single-ended and balanced outputs, so you can compare both.
I have owned both the Apollo and 840C. Both offer excellent sound for the money. The Cambridge, compared to the Apollo, is more up-front, and has a big, solid low end. I slightly preferred the sound of the Apollo (a little more relaxed than the Cambridge), but I found the Cambridge more user-friendly than the Apollo.
Here's one more option for you to consider -- keep your Cambridge 540 and get a good DAC. I really enjoy the sound of my Benchmark DAC -- in my system, better than both the Rega Apollo and the Cambridge 840C. You can order the Benchmark DAC-1 from Benchmark on a 30-day no-risk trial -- money back if you don't like it. The Benchmark has both single-ended and balanced outputs, so you can compare both.