I just upgraded my backbox speakers (two 5 1/2 4-ohm wired in series) and the cabinet speaker to a 8 inch (parallel wired 8-ohm) subwoofer with crossover. Did it according to instructions at http://www.dziedzic.us/wpc_speaker_replacement.html Next time I will just go with PinballPro or Flipper Fidelity with all the extra time I put in. In order to the subwoofer up to enjoyable bass levels, I had to turn the machine up to 15 volume. I keep most of my other machines around 5 volume. I then adjusted the backbox speaker volume lower to match via an L-pad. It sounds great as it could without upgrading to pinsound board. After 10 minutes or so the sound starts cutting out when the deep bass notes are played. It then gets worse until it is constantly cutting in and out. The heatsink on the amp is burning up. It does appear to be very well attached and doing it's job the best it can. I have LED's, so no extra heat there. The low pass crossover has a slightly higher impedance between it and the sound board (around 11 ohms measured with my el cheapo harbor freight multimeter). The subwoofer has around 9ohms. From what I read higher ohms is not as bad as lower ohms. I used the old thin wires that originally went to the old cabinet speaker, while I used 16 and 18 gauge for backbox speaker and l-pad. I did drill through crossover board order to mount it. Do not know if that plays a factor. Here is a pic of crossover and sub.

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At this point everything is done, modded, and installed. I read somewhere that bad capacitors could also cause overheating. Is that true?

Next step to repair, without adding a powered sub? Replace caps?

Bump! I am still wondering if replacing capacitors would fix overheating issue.