It you can catch him sober, offer to let him know in advance when and how late you plan to play, maybe he will be a little cooler about the situation. If he's a drinker, nothing wrong with that, then don't count on any consistency.I thought the suggestion about trying to catch your neighbor when he's not sauced and learn a bit more that might help you two reach some kind of reasonable compromise was a good one.
This is not a bad idea, especially if you have a chance to have a sober conversation with your neighbor. We don't know how your basement is outfitted currently, but sometimes even bookshelves with books at varying differences from the shelf edge can help.
That said, as others have pointed out, bass freqs are tough to tamp down.
Never underestimate the power of sound. I can't practice bass at my home without headphones. I can't really even play acoustic unless everybody, meaning family, it either in a good mood or gone.
