Upgrade speakers or DAC

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.

Speakers should be the first thing you base your system on and then eventually the last item of the upgrade.  All speakers have an unique presentation.  When you find that signature sound that you prefer, speakers become the foundation for which your stereo is built. At that point optimizing your system around those speakers to bring out as much of the reason you bought them should be the goal.  Upgrading speakers when the rest of the components and the room aren't addressed is just masking the deficiencies but they still exist.  I'm a great believer in if the first watt is crap then it's just amplified thru the whole system.  That applies to your source which in my opinion is where you should focus.  Simple things such as speaker placement and addressing reflective surfaces will make huge differences in the presentation. Have you considered a subwoofer?  Properly integrated it adds bottom but also opens up midrange.
My .02¢