SonicLink transmitter

I have a pair of Wave Eights which work fine for my setup. I would like to add a subwoofer but I don't want to buy another Wave 8 for this as £700 is a bit steep. I was hoping I could buy another SonicLink transmitter and connect it to a active subwoofer but I can't seem to find any online. All I can find is the HP TX01 for headphones only. I have tried the support page for parts which sends me links to UK suppliers but they cant help.

Or should I use the other output on my controller to plug in a subwoofer as there would be very little latency with the SonicLink speakers?