I replaced the cabinet speaker in two games, williams Space Station and Congo with this pair
https://www.amazon.com/gp/product/B00J2BK2NI/ref=ppx_yo_dt_b_asin_title_o02__o00_s00
They installed easily, cost $30/pair and made a big improvement with fuller sound and more bass. I replaced the speakers in the Congo head with some car speakers.
https://www.amazon.com/gp/product/B00NF5EQHK/ref=ppx_yo_dt_b_asin_title_o01__o00_s00
I found some useful info on this thread:
https://pinside.com/pinball/forum/topic/cabinet-speaker-size-and-wiring-williams-system-11
and here:
https://pinside.com/pinball/forum/topic/upgrading-speakers-with-car-speakers
Now I want to build a simple sub woofer for my Houdini but don't have a plan. Parts Express.com has all of the parts I'm just not sure what all I need. Does anyone have tips?