Ok, I am going to bring this thread up again just to get a little feedback on this .... and give the OP another avenue. Plus I like to go against the grain .... hahaha
So the WPC and WPC-S sound system is a bit lacking in the Bass department at stock levels, but I believe I have found a solution that is not very expensive and for the most part non invasive to your pins or space or power outlets.
Bride of Pinbot and Jack*Bot both have full range going to the cabinet speaker. On Jack*Bot the wiring is ready to accept this little mod. On BoP the wiring needed to be changed over to the same as Jack*Bot.
Jack*Bot Diagram (see Image Below); all Speakers are 4ohm .... BoP had to have the backbox speakers changed out to 4ohm to be wired in parallel to see the 8ohm load at the board and the Subwoofer on BoP was already 8ohm as that system stock used all 8ohm speakers.
On Jack*Bot the cabinet speaker was a really lack luster speaker but it actually worked quite well in the end.
So below are the 2 units that you can get from eBay
First one is a simple crossover .... $13 free shipping
And the other is a simple Class D amplifier ..... $15 free shipping
You will have to check the rating of your factory speaker and you might have to add a L-Pad to control the output a bit. It worked fine on my factory speaker but in the end I put in a Polk audio Low Profile Subwoofer in Jack*Bot and someone had already put a cheap Dayton Audio sub in BoP.
I realize that this is a little more time consuming and takes a little DIY capability, but for the combined amount of about $35 per Pin, not to mention the floor space and power outlet savings, I thought it would be useful to someone willing to get a little more for a little less.