Quoted from metallik:
I didn't add the amplifier components to the board, I tapped the line-level signal for the left and right audio from the board and ran it to separate self-contained amps, leaving the left amp on the board unused. This was back when it was easy to get amplified equalizers (car radio power boosters), so I ran the L and R signal to one car amp driving good coaxial speakers in the head. I think I may have just used a line-level converter on the cabinet sub speaker output and connected that to the other amp, which drove a much better replacement cabinet speaker.
Gotcha, too much work for me. I actually had a speaker from another BSMT 2000 game sitting on my shelf and adding it made a big difference. It was a perfect fit for empty speaker grill and took about 5 mins to get installed. It's just daisy chained to the stock speaker, the same way most Bally/Williams are.