I don't know TSPP, but my LOTR has a setting for volume effects. When the game gets exciting, like movie modes, or destroy the ring, the volume gets louder. It's really cool when noise isn't an issue, but when I play at night, and my kids are asleep, I turn the setting off, and the volume remains constant. TSPP may have a similar setting. I think everybody is right though, the speakers are the least of your concerns. I'd turn off, or turn down the knocker too if you're worried.
Another thought might be to simply knock on your neighbours doors, and say, "Hey, I'm a big fan of pinball. I own one, and I play a lot, but never want the noise to be a problem for you. I'll try to be respectful of the clock. Knock on my door if you ever want to play!"
I for one, would be thrilled to have somebody knock on my door, and say, "Hey, I have a loud hobby (carpentry, music, etc etc), but I don't want to be a PITA. Please let me know if it becomes a nuisance"
A little neighbourly communication goes a long way. My kids play guitar and drums, so I let the neighbours know there may be a lot of noise coming out of basement from time to time. I've had no issues.
I bet a foam pad under the machine would do wonders towards dampening what goes down through the floor.
Unfortunately, I find I play the MOST after 10 pm once my household has settled down, and my kids are in bed. Luckily they sleep on the 2nd floor, and my pins are in the basement.