I have an rdac with my squeezebox touch and it is a significant improvement on the squeezebox alone - much smoother and more detailed sound. Perhaps could do with a little extra punch ideally (IMO) though this may be down to my other kit.The Touch is suppossed to have a better sound than the Duet so I would hope that the improvement from the rdac would be even more significant in your case.
I have an squeezebox duetand I bought a Musical fidelity M1 Dac 100 pounds more than the acram rdac and there is a big improvment
also what was suprising was the USB connection for quality it sounded great so much so that I'm going buy a USB from wireworld to improve things further still, In addtion to that I've hooked up my DAC to my CD player and the qualilty is great it was a big improvment.
I can highly recommend The M1, the rdac I thought about because the USB is considered to be better than the M1 well if its better then it must be outstanding because the M1 is super,the bulid quality is great you feel that its worth a lot more than £400 pounds and my set up is now complete its taken a year and a half due to budget restaint but a good amp good speakers along with a streaming system all going through a Brilliant DAC which gives me optional connectivity.