you need the nonpolarized capacitors that are used for speaker crossover
parts. radio shack used to have them fairly cheap. buy several of the
largest value ones. one of the caps will go in series with the speaker.
now it will sound thin and tinny. (like a midrange driver....... thats
what the caps are for, to limit the bass to a midrange or tweeter
driver) now drop another cap in parallel with the first one. there will
be more bass available due to the change in capacitance. keep increasing
the cap value this way until there is enough bass to sound like you
want.