I'm using dual F8's, crossed at 60 with a pair of towers and thoroughly impressed. (keep in mind I have the option of using any Rythmik sub )
I can't put my finger on it -- perhaps it is the transient accuracy of the lighter mass cones, greater dispersion of smaller cones, or an improved upper bass response - but I have been partial to dual 8 designs since I worked on the M&K MX-70, MX-700 and a dual driver version of the VX-7 (which I had hoped would make it to market). All of these subs were sealed dual 8's...