Upgrade speakers or DAC

Both. The better the speakers are the bigger difference a better dac will make. The good thing is your amps and pre won’t need to be upgraded for the new speakers. is the cambridge an audible upgrade from the dac in the c2500?

Whether you upgrade, the streamer or the speakers first, you’ll be upgrading the other component soon after. 
 

I’d upgrade the speakers first but I would also say the DAC upgrade will be very significant as well. Also, in my experience, streaming DACs don’t feature the best possible streamer and a separate high quality dedicated streamer will beat the inbuilt network module. Something to keep in mind for the future. 

Unless your DAC is from 1997, a new DAC will make approximately ~1% of the difference new speakers (or treating your room) will make. 

Speakers will give you the most obvious difference, but in the long run, I think source is more important. 

Better speakers will get you a more accurate representation of the source signal you currently have, a more accurate source will give you the most accurate system possible, with any speaker.

I have listened to the SF speakers you have. Pretty good loudspeakers. But I would bet the jump from Cambridge to dCS would show you what your speakers are capable off. That can keep you satisfied for a year or two. Save and then jump to a much more better speakers than you currently intend to get. Not sure if you have room treatments. They will make a positive impact when placed properly.