Trying to build a bass trap for an outdoor speaker might be the very definition of the phrase "an exercise in futility".
- You might try investing in a graphic equalizer and attenuate the low frequency range. Or turn down the bass on your source device, pre-amp, or receiver. Beware, this can or will leave your music somewhat less satisfying.
- You could move the speakers closer to you and reduce the volume.
- You could reduce the volume.
But bass traps for out side aren't going to help. You can hear for yourself what the problem is. High frequency's are very directional. They drop off quickly and are pointed away from your neighbor; that's why he doesn't hear them. Bass frequency's are long wave length energy and omnidirectional. Containing them with a trap on the back side of the speaker does nothing for the rest of the dispersion area that radiates low frequency and this low frequency will still find its way to your neighbors ears.
I agree with one of the above posters, a mono feed for these types of speakers usually produce better results for over all sound quality outside, but a mono system will do nothing to control the bass output. Nor will a 70v system. While there are certain advantages to a 70v system, there doesn't seem to be any problem based on your question and description of your system that a 70v constant voltage system is going to improve or impact. I see no benefit in a 70v constant voltage system for you in your current configuration.
The basic advantage of a 70v constant voltage system is the ability to use multiple speakers over long distances for distributed audio. This is done by using a step-up/step-down transformer to minimize resistive power loss. The other benefit is you don't have to worry about impendence matching at the amplifier, it's more easy to provide the amplifier with a matching load; you don't have to worry about connecting multiple speakers wired series-parallel.
But back to your problem. Another possible solution for you is to install some additional speakers, distribute the sound if you will, so that when you move around you maintain a comfortable listening level at a lower overall volume. If you were to do something like this, then a 70v system might have some advantages.
If you go with a 70v system, it sounds like your current speakers already have a transformer installed. You will need a transformer for your amplifier or buy an amplifier that supports 70v systems. I would recommend a multi-zone amplifier that supports either or 70v and low-z as well as individual channel gain controls.
Here's some links to give you some background information for 70v constant voltage systems:
EDIT: I took a look at the Boston Acoustics site and the speakers you reference. Looks like 70v operation is an option but you need to buy the transformer form Boston Acoustics for this to work.