There is a persistent issue that affects some Spike-2 machines. It does not seem to be dependent upon the title since it has been observed since about Iron Maiden (which had a CPU node board revision after the LE run). However, it has also been loosely reported in Star Wars.
Below are two key PinSide posts describing the issue. 1) below is the start of a discussion, 2) having more specific language describing the sound issue. The discussion trails off after 2):
1) "Weird issue... Was in the middle of a game when it sounded like the music cut the bass output in half. Nothing weird about the game, was a normal shot up the middle, maybe some pop-bumpers, but not in any mode. Got underneath and listened and the sub is still outputting sound. Just didn't have the deep bass. Tried turning the machine off and back on. No luck... any thoughts?"
2) "Like clockwork. Halfway through game 2 it happened again. Since its fresh on my ears I can better describe. It almost sounds hollow all of a sudden. All 3 speakers had sound going through them. Started in ball 2 and lasted rest of the game. While typing this it went back to normal on its own about 5 mins into the next game (have it sitting started with ball in shooter lane).
Stock speakers and highest the game has ever seen is 20.
It makes my ears hurt the same as of I have to much treble coming out of my car speakers but it doesn't sound high pitched."
This issue is specific to Spike and has nothing to do with the PinWoofer system.
We have one read-point (on a highly repeatable Led Zeppelin) that the Spike-2 Node Amplifier is the culprit since adding our expander board delete kit has solved the issue (i.e. now taking the audio input to the GT amplifier from the line out board). This is one case and if other cases present themselves we'll update this blog entry.