PE doesn't sell a 4ohm 6.5" Vifa mid-bass anymore, or I probably would have gotten them. The 7" Vifa's at speakercity.com are an additional $40 + shipping and may have required a little metal trimming the way I mounted them. So, I guess it's $$$ and ease of installation.
Anyways, these 5" Vifa's have more mid-bass kick being driven by 25w than my former 6.5" Focals driven by 80w.