upgrade Dynaudio Special 40 to Kef Reference 1

The real question is what are you trying to do?
Changing the speakers will be an apples to oranges change.the Kefs are more transparent, more dynamic, and less colored, they also go lower and sound bigger, they also image with both a large and well focused sound stage.

The Dyanaudio loudspeakers are richer in the midrange, with a bit less treble detail, they do sound big for a small monitor.
Changing your electronics or dac will or may bring out more from the loudspeakers.
Op what makes you think the Yamaha amplifier is any better than the Hegel Just the contrary the Hegel's clean, dynamic sound would be excellent with the Dyaudios, the Yamaha may actually make the system sound too sleepy and less exciting.
The best thing you can do is to figure out for yourself what I am trying to accomplish? If you are craving more detail, a bigger sounding loudspeaker and deeper bass, then the Kef is for you,

If not stick with the Dyaudios and maybe you need to look at an even better dac or source upgrade.
The other thing is that there are a zillion great loudspeaker choices even in a smaller room a set of compact floor standing loudspeakers may be the ticket.
Dave and TroyAudio Doctor NJ

Kef and Dynaudio sound totally different, in general. I prefer Dynaudio sound but that’s just me. Dyn’s are typically harder to drive (more difficult impedance curves, lower sensitivities). Kef is good too though, especially if it is important to you that the entire room sounds pretty good, as opposed to being optimal in one listening location. Kefs might be better than almost any other brand in that feature. Really good choice if H/T a part of your system and whole family watches movies together, for example (foregoing not at all to suggest Kef not good for music, it is as well).

After owning all the above mentioned I would buy Piega coax 311. 
A keeper 

You need to drag your amp to some stores and LISTEN.  Only then can you tell whether there is truly and improvement or not, and whether that improvement is worth it in monetary terms.  Everything else is scratching a mirage-like itch.

Kren’s point about KEF Providing more consistent sound from different listening points than most including Dynaudio rings true from my experience with Dynaudio Contour monitors versus KEF ls50. The Dynaudios were very good but seemed to require more care in exact positioning as well. Dispersion with the KEF UniQ driver seems top notch. I often listen from my desk located between the two speakers and the KEF ls50s retain their character particularly well even there.