There are a number of buyout speakers at PE that look interesting for different projects.
For extremely cheap bass to add to an OEM system these
6x9s have an 8mm xmax and a .8 qts indicating that they would work pretty well in an IB installation. A pair of these on the rear deck would be hard to beat as far as cheap bass goes.
For someone thinking about doing and interesting speaker project like a line array or a very dynamic multimedia speaker, here is a stellar deal.
2" extended range driver for 90 cents a piece. A series of 32 of them in a 64" x 2.5" x 5.75" (internal dimensions) with 4 2" ports 2.5" long will give you greater than 100dB from 120 hz on up with only 4 watts. That would be a little under 70.00 with shipping and you could blow away most multimedia speakers. The response is pretty ragged in a few places but a couple of notch filters will cure the peaks. The dips are another story, but the response doesn't look any worse than something like a Bose product.
Another potential line array candidate is
http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=269-568 which is a 4" wide range driver.
16 of them in a 2 cu ft sealed box should yield some impressive results.
In either case a subwoofer would be necessary at some point, but for the size of the drivers, the bass should be acceptable for temporary duty.
For a budget subwoofer for a moderate size ported box, this
Pioneer 8" DVC sub looks good in a 1.5 cu ft box tuned to 42 hz (3" port 5.25" long)
