Cannondale Supersix Evo headset woes

Does the new headset cap physically touch the headtube, if so you need some microspacers between the split ring thing and the cap.
If this isn't the case then make sure you've got enough room for the compression plug to actually compress the bearings as suggested above.

This may be obvious but have you checked that there is 3mm or so of spacers or stem showing above the end of the cut steerer before inserting and tightening the headset compressor bung? Sticking a 5mm spacer above the stem then refitting will clear things up either way.

You may have already ruled this out of course in which case I can't think why it would need a micro spacer but can't suggest anything else.

Cheeky question though, can you tell me the outside diameter of the 25mm spacer? I've got a non Evo Supersix and it has an even bigger spacer on it and am wondering if the Evo one would fit. If you're too vexed by the headset issue tho I fully understand!