I've owned Resolution and Absolute Reference and have heard Designer's Reference. Among the newer generation, I've owned Tesla Accelerator and have heard Acoustic Reference. I'm going to generalize here and say from Resolution Ref to Absolute Ref to Designer's Ref clarity and focus increase incrementally as does neutrality. Resolution and Absolute Ref would be considered by some to be a bit more diffuse sounding/warmer, somewhat stereotypically tube-like and maybe easier on the ears, while others might characterize them as a bit fuzzy or colored sounding, particularly from the lower mids on down. The Designer's Ref cleans things up somewhat, tightens musical outlines and moves a little toward the more stereotypical solid state kind of sound.
Quite a few owners of the newer Synergistic line claim very significant improvements in all key areas---vivdness, detail, imaging/soundstaging, neutrality, air, harmonics, etc.---all the way up to the top of the line Tesla Apex. Again, in terms of direct experience, I can only speak for the Accelerator(owned) and Acoustic Reference(auditioned IMS) models. Though clearly impressive, they were not my "cup of tea". After a while, I found them not to be as relaxing sounding as I would have liked. Of those in the older series I initially referred to, I would consider the Absolute Reference both better suited to my tastes and the easiest to live with.
I cannot emphasize enough these were MY impressions gathered from MY system(s). You MUST sample---mainly in your own system---these or any other cables you're strongly curious about and/or seriously considering purchase of. Best of luck.