On the Ribbon Dipoles, the 6.5" WCF Megawoofer, which first arrived May 2004, is standard.
The FST option is $800 MSRP/pr (this is because there are two per speaker, $400 per pair x2 = $800).
Personally, I've never compared Auricaps/SC vs. without in that speaker in that application, & I doubt anyone has. I suppose Brian could call me over next time he builds a pair with the options & I could A-B with a standard pair.