Which lay flat (157 degrees) car seat is tested and fits on the Reef chassis? I was looking at the Maxi Cosi Marble but don't know if it has been tested and approved on the Reef frame?
The brands often test only a few car seats and prefer their own. If not a small stroller, though (which the Reef is not), you should be ok with any car seat that fits the adapters. In this case, I would not be worried about the Maxi-Cosi Marble, the Joie i-Level, The Kiddy Evluna i-size or the Cybex Cloud.
If still in doubt, be sure to write to Silver Cross customer service, they will have the most updated info, I believe. But I am pretty sure that with Maxi-Cosi adapters, abovementioned car seats will be ok.