Need a passive low bass unit for Anthony Gallo Micros

Hi everyone
I need to build a small passive low bass unit (subwoofer) for my Anthony Gallo Micros. (Full range 4" driver in a sphere)

The one that is sold with the package is a sphere with an 8 inch Peerless driver (dual voice coil) but it costs 300 bucks
I think I can build something that will work for cheaper than that ya think?
The Micros are 89db sensitivity and 8 ohms ea.
Bass response for the little guys are slim to none so I was thinking about a 125 hz crossover point?

My original plan was to find an 8" driver (dual voice coil) that will work well in a small sealed or vented enclosure and cross it over to the Gallos via a first order Butterworth at 125 hz.

Would this be a good plan of attack?
(of course the sesitivity of each voice coil would match that of the Gallos, or is that really important?)

Any suggestions would be greatly appreciated