There is no quick or inexpensive way to reduce low frequency leakage to adjacent properties. Doing so requires extensive structural work and typically the creation of secondary walls or floors. A first step is to talk to your neighbours to establish the extent of the problem and the degree to which the leakage affects them. If it is a significant problem then a change of speakers is probably necessary. Ideally, you will find a speaker that sounds lively at "lowish" listening levels.