What is the size of your open? I had a ‘01 43 open and If I remember correctly the speakers were wired directly to the sub-woofer which has the amplifier built in. There should be an umbilical/control cable that runs from the sub to the Boase lifestyle head unit, located in the entertainment center. Sub-woofer was mounted under the salon floor, port side I believe.
I had a similar set-up to JClark on my ‘99 4000 Express. Someone had removed the entertainment center, so it was all a bit of a mystery until I accidentally found the subwoofer behind a false panel one day. Then it started making sense.
I’m doing some research, the original sound system by Bose was bulky and near-impossible to fine a replacement. The speakers aren’t really useful with any other system, so I removed the speakers and the subwoofer and installed a soundbar under a new flatscreen. Added bonus was that it has Bluetooth so I now also have music down there if I ever want it.