This is a personal project that started with the idea of that these bluetooth speakers needed more bass. The idea was to add a signal level output that could then be amplified while retaining the Boom's bluetooth linking system.
The waterproof design of the boom made it a little bit of a hassle to disassemble the unit. In the first picture you can see the mounting system for the top caps with seems a little bit delicate as the connections are all based around the leads making contact while the unit is closed. Although this design does make disassembly and reassembly easier i am not a fan because of the chance for eventual vibration damage, but over the life of the speaker it shouldn't matter.
The next challenge was finding an output that would be suitable to amplify. The first idea was to use the bluetooth outputs but this wasn't possible as the motherboard does some processing to maintain time alignment of the speakers by speeding up or slowing down the output. Which is very clever but means that another access point need to be found. The simplest solution was to pull right off of the speaker. This was only possible because the impedance of the speakers matched a normal headphone output. Likely UE designed the Booms to use high quality headphone speakers that were "Off the shelf" components.
The only concern with drawing off the speaker is that it has the possibility of over driving the amplifier by providing a lower impedance than it was designed for. It would be impractical to wire the output and speaker in series because the speaker when not connected to an output would be an open circuit. This configuration would raise the impedance and would be safe for the equipment overall but during normal operation it wouldn't function properly. Therefore it had to be wired in parallel. Thankfully the speakers were 32ohm and generally all other headphone amplifiers act as 16-64ohm loads, meaning that the change would not be too dramatic.
A mono 3.5mm minijack was added and ran through the chassis and out the top before the unit was sealed after initial testing. This was then paired to a 100w amplifier and a 10in boxed subwoofer. The amplifier is low passed at a crossover frequency of 200hz at -12db/octave, which is a little high but not a problem as the Boom appears to have a slight high pass at around 50hz at about -6db/octave so a little bump into the mid bass makes up for the lack of very low end.
The results are better than anticipated, the subwoofer is very efficient and even with only 100w can fill the whole house with bass. The unit time aligns with all the other units present and fills out each individual speakers range. It is a great example of the non directional nature of low frequency audio as in other speaker in the area that is linked appears to be the source of the bass.