Listen, sound patterns travel in the weirdest ways. My house is on a hill and there is a college a block away. Every night my house was shaking from bass and I was like "what the hell". I am thinking the kids were cranking up their bass so I ventured out on a recon mission.
After a few nights of visiting the parking lot area I did realize it was a few kids. But their music was kind of loud but really not bad at all. The sound waves just climbed right up the hill and bam hit my house. I just asked them to move literally 30 feet over behind a tree and have never heard them since (and they still hang out there).
For your issue you have to get rid of all of your power for a night to make sure its not you. I believe you when you say its your neighbor but check yourself first. I did have a roommate in college that slept with semiloud music on all the time.
As for traveling through pipes... I doubt it. Unless their system is setup right next to the sewar lines or main stack really there is no way. Most bass back echos. So if they have a system that is facing away from your house in their room facing you, that would make more sense.
I would talk to the neighbor. Your options for making this sound go away on your own probably involve a tree or fence which I am not even sure is possible.