DIY Subwoofer

This might be more ambitious than your friend want to undertake, but you can try this site:
http://www.royaldevice.com/custom3.htm

Way Abstract- Undertaking is the right word for that subwoof!

If he has any (reasonable) woodworking skills, then he can look into kits, one of the best being ACI Titan. You can find info at http://www.audioc.com/

Here is a comparison of several different drivers in different alignments.

http://www.zebra.net/~mvance/projects/bass/Twelves/index.html

When looking at any woofer spec's, ALWAYS take a look at the impedance at resonance. The higher the impedance, the more "out of control", "bloated" or "one-notey" it will sound. While this is not nearly as critical for HT use as it is for reproducing music, it is something to consider. If your interested as to why this occurs, let me know and i'll do my best to explain it in "English". Sean
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I got excellent bass using the NHT 12" driver ($149)and madisound's 91 liter enclosure which I lined with a another 3/4" layer of MDF to make the walls 1.5 inches thick. Stuffed with fibreglass, it gives deep powerful bass. You need a solid 250 wpc to drive it though.