The issue...I had a 5.1 Home Theatre in a Box speaker set that was of good quality (made by a company that specializes in speakers) that had a built in receiver/amplifier. This system worked fine for several years until I got a new 7.1 receiver. All of the other speakers from the system hooked up as you would expect to the new receiver, except for the subwoofer. Seems that today's surround receivers require a powered subwoofer as part of the system. The subwoofer from the HTIB was in perfect condition and sounds fine as is, so the solution became to add a separate amplifier to drive it.
Step 1: Remove back of subwoofer
My sub had the receiver/amp built in to the back of it. I removed these parts since they weren't being used and found the two (+ and -) wires to the speaker.
Step 2: Buy a subwoofer plate amplifier
I found a 25 watt subwoofer plate amplifier that was in the $50 range and had positive reviews. You can get much more powerful ones but the price goes up accordingly. That said 25 watts seems to be more than enough for my purposes and for this sub. It is audible from outside of the house when cranked and can go far louder than is comfortable to actually listen to.
Source (US): http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-782
Source (Canada): http://www.solen.ca/pub/cms_nf_catalogue_fiche.php?id=2366&recherche=&numRows=&manufacturiers=&niveau1=1&niveau2=2&niveau3=41
Step 3: Build a box for the plate amplifier
The plate amplifier I purchased was the exact width of the case for my subwoofer, but the cavity was not quite deep enough so I built a little box for the back to add more space. I pre-drilled some holes for the screws and slapped some old black paint onto it. I used some leftover MDF that was the right width. I didn't take to much time with this as the subwoofer is sitting inside a component niche in my living room and can't really be seen.
Step 4: Add some cushioning material to the box
Even though the box is being screwed on tightly I added a bit of double sided foam tape to the back edge of the box just to be sure that there would be no rattles down the road.
Step 5: Screw box to back of sub
Pre-drill and countersink the holes for the screws, then screw the box securely to the back of the sub.
Step 6: Join wires from amplifier to sub
I soldered the wires together and used heat shrink tubing to make sure they stay put. Also did not want to use any mechanical fasteners that might rattle.
P.S. - Test the connection with your system first before you solder. I made this mistake even though I joined black to black and red to red...the polarity was still backwards for some reason.
Step 7: Screw plate amplifier to box
The last step is to screw the plate amplifier to the box, hook up to your system and then enjoy...
I estimate that this project saved me $200-250. Seems that even cheap powered subs are around $300 in Canada.