There are a couple of other approaches to take.
The volume a speaker puts out is defined by its efficiency, not its resistance.
The stock speaker is quite inefficient for a 12" at 87.2dB. I replaced it with one from MCM Electronics with an efficiency of 97dB - that means that it produces almost 10 times the sound level from the existing amp. So if you were getting 5W, this speaker makes it sound like 45W.
This one has similar specs, but is cheaper right now.
http://tinyurl.com/c4tz7rw
The Fs (low frequency resonance) is good enough.
On the backbox, we made spacers and replaced the existing speakers with car units. This was way too loud, so I put the cap back in. It really needs a ceramic resistor in there, I need to set up a pot and test it.
Here's my pictorial on the backbox:
http://mrtn.smugmug.com/Other/2012-07-06/24018064_vJNDHk