Subwoofer for single driver Fostex Voigt Pipes??

by far the best subs for single driver speakers are TBI. They use 6-8" drivers which are incredibly fast and will be able to keep up with the fostex. they are capable of going down to 18hz. check out the reviews at 6moons (TBI used to be called VBT)
http://www.thoroughbass.com/

Cain & Cain make the "Bailey" sub to go with their "Abbey" speaker (which uses a Fostex driver). That might be a good place to start. Also (and in general) REL subs seem to work with just about everything. And, finally, Omega speakers use Fostex drivers--and they have a recommended sub listed on their website. If none of those ideas work for you, try doing a search in the "high efficiency speaker" section of audioasylum. (www.audioasylum.com)

I have added an infinity Intermezzo 2.1? With 800 watts D class Bash amp. It has a built in parametric eq....I run this parallel with the speaker output of a pair of back loaded Fostex 208 Sigma drivers with great success. I got it for about 1K, in a close out from Audio Advisor.