Nice setup. I also have some experience with the Vifa drivers. In the proper enclosure you can get -3dB output @ 42 Hz or even lower.

You can buy the Vifa 5" Midbass from Parts Express for $29/ea. Other Vifa/ScanSpeak components are available as well.

I had a pair of these at the rear of the doors of my ZX2 as extra bass drivers. They filled the passenger area quite well. Eventually I put them in computer aligned boxes with a pair of Vifa tweeters that are used as satellites for my home TV. Combined with a nice sub these are awesome speakers for the $$.